Not sure if cat could have a disease that makes her gain weght?

So last year I rescued Mama and her kitten because it was getting very cold and had some snow already. She was extremely skinny and had worms and fleas. Took both her and kitten {Little Girl} to the vet and had them give them a physical and vaccines. They also gave Little Girl a dose of revolution along with Mama. Kept them separated from my other animals for about a month with gates separating them {six other pets}. When I started letting them out I just fed them as I feed my other three cats {which is grazing as they want to} I started noticing that Mama was putting on weight very quickly so I now feed on a schedule and watch what they eat. None of my other cats including Little Girl has put on any extra weight but Mama keeps gaining. I asked vet for some advice on weight control food and I also try to get her to play but she really doesn't want to play with anything! I have lots of toys including laser, catnip, fishing toys, one of those rounds things with ball in it, and I try to get her to chase a toy on end of string while I pull it. I'm beginning to think that there is some kind of health problem, but have no idea what it could be. Blood tests haven't shown any irregularities so wondering if anyone might have an idea.