First thing you need to do is rule out a UTI or some other underlying medical problem through you veterinarian.

I have a senior cat, Mollie Rose, age 15 who is very thin (I attribute it to age) and I keep my eye on her for anything unusual. She is still drinking alot of water and using the litterbox quite a bit as well. But my vet said this is normal for a cat her age.

Please take your kitty to a vet and have her checked out. It'll definitely give you peace of mind.