There are so many things that it can be, you really need to take kitty to the vet so they can help you to figure it out. It might be that kitty ate something that is making it sick, it could be something with the stomach, an injury that you don't know about or poisoning. Please take it to the vet as soon as you can. Hope this helps.
Tiggersmom is right, get kitty to the vet ASAP! Cats can lose weight very quickly, and they don't have any weight to spare. A male cat I had was chewing on a new toy I had given him. The vet ran through a list of anything new I had given him, or he had eaten, etc. When I mentioned the toy, she said it was probably toxic. He lost 2#s in a very short period of time. He recovered very quickly, when we found the cause, and he was treated properly. Good luck with kitty.
I was once told that if a cat has a stomach ache, they eat grass or plants to help themselves feel better. However, I am not a vet so I am not qualified to give you the right answer. PLEASE, take your kitty to the vet clinic.