Sorry for taking so long to get back with you. I had to take the time to look up the exact numbers to answer your body fat question. On the CM, yellow is a less-fertile type CM, and it sounds like it's your Basic Infertile Pattern when you're not ovulating. That actually sounds very similar to my BIP in my postpartum cycles. It is most likely completely normal, but if it's accompanied by a bad smell, you might want to get checked out for the possibility of an infection. Just smell your panties or enlist your husband's help. If it's not obviously bad, you should be fine. Sweat doesn't smell great, so don't worry about a little bit of BO. I'm talking about a truly bad smell. Really, it's probably just your postpartum BIP.
On the body fat, my first response is that absolutely, that could be your issue. It is well known that losing a large amount of weight can be associated with infertility. Fertility hormones are fat-soluble, and if your body fat is too low, you will be infertile. This is actually one of the natural ways of controlling fertility since a woman can be healthy without enough body fat to conceive. The numbers are a little bit confusing to me, though. The recommendation that NFP international and the Couple to Couple League use is to aim for a weight of 100 lbs for 5 feet tall and then add 5 lbs for every inch taller. At 5'5", that would put you at 125 lbs. However, based on a personal discussion with Marilyn Shannon of CCL (author of Fertility Cycles and Nutrition), it is her belief that due to different body types, some people may find their optimum fertility approximately 15 lbs more or less than that estimate.
I think that body fat matters a lot more. Most women need at least 20-22% body fat to be fertile (this is the statistic I had to look up) according to The Art of Natural Family Planning 4th edition, though most fertility specialists believe this number to be lower, closer to 17-20%. I personally think that the TAONFP estimate is probably more accurate, and I have a personal theory that the amount needed to conceive may be more while breastfeeding (since it appears to be possible in many situations to override natural breastfeeding infertility with extra body fat, and personal experience of being infertile in that range). It could be quite possible that this could continue for a time after weaning as well since it takes some time for the hormone levels to return to normal.
Another practical piece of advice that I have heard is to aim for your pre-pregnancy weight that allowed you to conceive before. After all, it worked before, right? Since either too much or too little fat can cause fertility problems, this isn't bad advice.
Given all of that, I would guess that being a little bit over the 125 lbs estimate is not your problem. You conceived at far more than that. However, I'm not sure that gaining back all 20 of the lbs you lost is the best idea either, since that would put you far over the 140 lbs estimate for the maximum amount of weight you should need for maximum fertility. If you can estimate your body fat percentage any more accurately, that might help to aim for 23-25%, though you may already be in that range. (While nursing, I wasn't able to conceive until I had about 23% body fat. I had an inadequate LP at 21-22%.) If weight is a problem for you, I wouldn't guess that it's much of one, but it definitely might be worth trying to gain a few lbs and see what happens.