Everyone has a set weight that their body is comfortable at. I am 5'9" and my body likes being about 145 and I have to fight for anything less than that.
Does your diet change to contribute to the weight gain?
This is one of several reasons I'm not a fan of the no/low carb diets, the weight comes off pretty easy but it comes back much easier

I know it's a cliche' but it really is a lifestyle change. To maintain weight loss you must exercise and it needs to be hard enough that you are burning fat and building muscle. The more muscle you have the more fat/energy/calories your body burns even when you're not working out.
The first rule of thermo-dynamics is more calories consumed than burned = weight gain. In order to lose and maintain you need to create a caloric deficit every day. You need to burn more than you eat. If you consume 2000 calories per day but burn 2200 per day you will start burning stored fat to make up for the energy deficiency.
Keri