Sudden weight gain?

My cat daisy, sheâ€[emoji]8482[/emoji]s around three years old. She lives with two other female in-door only cats, one male in-door only cat, one female indoor/outdoor, and a host of other animals. On July 7th, we left on vacation, and returned a week later. Since then, we have not been home, due to our home being extremely hot. The cats are coping, they donâ€[emoji]8482[/emoji]t seem to be nearly as stressed as we do over the heat. We have spent time away from home, cleaning up our home after some unhappy kitties trashed it. (throwing EVERYTHING off of ANY surface they could) The cats seem to miss us, and Daisy has seemingly gained A LOT of weight. She was my smallest cat, at 6lb and three years old, but now, she seems to be my biggest. Also, her stomach seems fairly firm when sheâ€[emoji]8482[/emoji]s stretched out. (She almost feels like sheâ€[emoji]8482[/emoji]s pregnant! Which is impossible, sheâ€[emoji]8482[/emoji]s fixed!) She has had problems on and off with her stomach. She vomits up her food, and fur, which is still undigested, or partially digested. We now feed her, and the rest of the cats, Science Diet Sensitive Stomach. So Iâ€[emoji]8482[/emoji]m wondering— Is there anything I can do to curb this eating habit? And how can I slim her down?

Oh, and, none of our other cats have had this problem, either.

PS: i'm not sure if this is the place to put this, sorry if its not!

Take her to the vet.