She might have a blockage somewhere from swallowing something she can't digest. Does she ever eat weird things around the house? Unfortunately, it's hard to say what exactly is wrong because those symptoms could be just about anything.

I will also warn you that if she isn't eating, and she starts to lose weight rapidly it's not good, because cats can't really metabolize their own fat. It collects in the liver and will shut it down, and kill them in a fairly short period of time, especially if the cat was overweight to begin with. So you might have to break down and take her to the vet.