Disclaimer: I'm not a physical trainer or nutritionist, just a fitness junkie who has been into it for many years and reads a lot about diet and exercise. I run twenty miles a week and take an intense fitness class twice a week to stay in shape, and maintain a very clean diet while breastfeeding a ten-month-old. Right now I'm pretty much at my goal weight and just working on meeting my own running goals and toning up.
When you say your belly is getting bigger, do you mean you're steadily gaining weight? Or do you mean it feels like you're looking more and more preggo while the rest of your body stays mostly the same?
If you have diastasis recti, which is the thing where your ab muscles separate and don't come back together after pregnancy, doing crunches or other ab-focused exercises can actually make your belly look rounder and more "poochy". If you think this is what's happening, it'd be worth checking out Pilates and/or physical therapy to figure out ways to strengthen your core and help repair the DR. Surgery is needed for extreme cases, but my postpartum Pilates instructor (who's also a physical therapist) told me that in her many years of working with post-pregnancy bodies, she's only seen one or two people who couldn't fix their diastasis issues with a belly band and regular PT or Pilates work. There are some great Pilates videos available on Youtube, or you can order some from Amazon, or just take a few postpartum classes (I'm sure your gym can help you find some).
As for why you might be gaining weight or not losing the extra weight you have, the short and most annoying answer is that you're consuming more calories than you're burning. More calories going in than calories going out = weight gain. Assuming you're not pregnant or on medications that drastically slow the metabolism, what's probably happening is you're overeating. A 12-ounce regular soda is about 130-150 calories; I don't know what size mochas you're drinking, but that's a lot of empty calories right there. I was shocked to figure out that a large unsweetened skim latte still has 200-plus calories! When I was having one or even two of those a day, I thought I was being "good" but really, it was a huge calorie surplus that I didn't need. A full fat or flavored coffee is a huge source of empty calories and even just a few a week can really add up.
The way I've been successful at countering this sort of accidental overeating is by keeping a food diary from time to time. I always read labels to find out what a serving size is and how many calories are in that serving, and when I feel like my diet could be better, I document every single thing that goes into my mouth. This keeps me accountable and on track. It was also really shocking when I first started doing this to realize how tiny a serving size actually is compared to what I always thought it was. Most people have "some chips" without counting out like "about 16 chips" or weighing out 1 ounce, which is what the label will describe as a serving size. But eyeing it or having "just a few" is how we overeat, especially with junkier foods. Even organic baked corn chips have a ton of oil and salt, for instance, and it's easy to eat way more than one serving size if we aren't paying attention. If a package of "healthy" cookies has 10 servings in it and each serving is only, say, 80 calories, it's still overeating when you don't realize or care that a serving is one cookie and eating four of them just put you at about the same caloric value as a healthy lunch... I used to do stuff like this all the time before I started keeping track and holding myself accountable.
You might want to get a cheap kitchen food scale to weigh portions. You won't have to do this forever, but it's imperative that you learn how much a serving actually is versus what you think it is and usually eat. For example, a 5- or 6-ounce serving of meat is about the size and thickness of a deck of cards. How many Americans eat much bigger meat portions than that? Most of us, I'd bet. Keeping a food diary and sticking to strict portions sizes allows you to control your caloric intake, which leads me to my next little mini essay.
To lose a pound of fat, you need to burn 3500 calories. So to lose a pound a week, which is a healthy and sustainable amount and less likely to lead to the yo-yo up-down effect than crash dieting, you need to burn 500 calories more per day than you take in (-500 x 7 days a week = -3500 calories per week = 1 pound of fat lost). Depending on a number of factors, like your current weight and height, your activity level, whether you're breastfeeding, etc., your caloric needs number will be very specific to you. There are lots of caloric needs calculators online; I like this one because it gives you several options. Don't starve yourself, but don't let yourself overeat either. If you want to have a splurge day once a week, keep it sane. You can easily undo all your calorie cutting from the week before with a single restaurant meal! Avoid restaurants if you can, because portions are way huge and almost all restaurants add massive amounts of fat and salt to their food to improve flavor.
Weigh yourself once a week to stay on track; try to weigh yourself at the same time of day as your weight can fluctuate wildly depending on when you last ate and other factors. It can also fluctuate from day to day, so weighing yourself daily might be more discouraging than just a single weekly weigh-in. And remember that muscle really does weigh more than fat, so weight is way less important than body fat percentage and measurements like waist circumference. I'm now in the shape of my life and actually weigh a little more than I did when I was at my skinniest after DS. But I'm a pants size slimmer and trim and toned instead of the dreaded "skinny-fat".
Going to the gym three to four times a week is great, and you should keep it up! But weight loss is almost entirely dependent on what you put in your mouth. Overall fitness requires a controlled diet and exercise and being healthy is (I hope) your ultimate goal. Being healthy and having a body you enjoy looking at may require some weight loss and specific types of exercise, but those are both achievable if you're willing to hit the gym a few times a week and really work at it.
My apologies if you already knew all this information; it sounds so simple but until I really accepted that every single calorie I put into my mouth counts, I had trouble shedding that last 10 pounds of baby weight. Once I started a food diary, it fell off almost effortlessly. Low calorie, nutrient-dense foods are easy to find (grape tomatoes, carrot sticks, apples, berries, bananas, the list is endless) and will keep you feeling fuller longer than junk. You don't ever have to be hungry while you're cutting calories (though you may feel hungry from time to time, which is perfectly normal) if you have access to lots of fresh fruits and vegetables and are willing to drink a large glass of water instead of a soda or something similarly fattening.
One last thing that really helps me is to think of food as fuel. Instead of telling myself I'm "on a diet", I realize that "diet" literally means "what I eat". It's a little mind trick that has totally changed my relationship with food for the better. I don't reward myself with a dessert or a glass of rich beer; I just factor it into my daily calorie allowance and allow myself to eat it as long as I cut calories somewhere else. Instead of seeing this as depriving myself, I just look at it as a balancing act. That slice of chocolate cake will be delicious! But it will also cost me the cheese plate I was planning to have for dinner...and that's okay. I'll have veggies, cottage cheese, and fruit instead.
I also noticed that as I got into better shape, heavy, fattening foods became far less appealing. I'd rather snack on raw vegetables than something like chips and salsa, which I never thought I'd be able to say. The old saying "you are what you eat" is truer than we may realize.