Hi there,
I'm not a vet, so please don't take my word as gold, but I've lived with cats pretty much my whole life. Sometimes they go through phases where they will eat A LOT, and if they are an indoor cat (which all mine were) will become more sedentary and gain weight. Just like humans, some days they will be fussy and not eat as much, and that's ok too. With that being said, if they go through prolonged periods of not eating, losing weight in a short time span, not drinking, not using the litter, hiding, you need to see a vet PRONTO. A few years ago my baby boy (I call him that, but he's a senior cat) almost died from Pancreatitis, and those were the signs. Any "odd" behavior that just does not feel right (trust me, your instincts will kick in) should be brought to the attention of a vet. Here in Canada, you can go to a University where "practicing" vets (not certified yet), still studying under Certified Veterinarians, will offer a fair/discounted rate for any medical examination. Perhaps something like that exists in your area to save $$.
Oh, and I meant to ask, what kind of food are you giving her? I find that cats tend to eat more of the cheaper-type food found at grocery stores as opposed to the higher quality food found at specialty pet stores/vet offices. My vet told me the "cheaper" stuff is like McDonalds to cats! It tastes so darn good, and only fills them up for a short period, so they tend to eat more, gain weight. After my cat almost died, I had to switch him over to vet quality food (which is a hefty $$, but necessary). HE HATED IT. Protested for a few days, but in the end they won't starve themselves

. Now I'm not saying to run out and buy vet quality food, but if you are feeding your cat a $10 bag of food, perhaps spend a
little more and search for a higher quality type food. Yes, she might protest it. That's ok. Like my vet said, they won't starve themselves and will give in eventually.
Sorry I wrote a long post

. A lot of content there. Good luck and take care! Wishing you lots of healthy long years with your kitty

.
Natasha