I would take it to the vet, it could be that the cat is diabetic.

The safest bet is that its dying.

It sounds like it may have a kidney problem. Take the cat to the vet or call them and tell them the symptoms.

Sounds like she may have diabetes. That is what has happened to both my cats. They drank a lot of water and had lost weight. Take it to a vet and they most likely can prescribed something to help your cat deal with it. Good Luck!

time to take the kitty in to the vet.
unexplained weight loss & the need to drink more are always signs of things going on, like diabetes, kidney disease, thyroid disfunction, etc. there are a number of things that could be causing these symptoms, and only bloodwork will be able to pinpoint just where the problem originates.
and it doesn't necessarily mean the cat is going to die no matter what. some of these things are quite treatable. your kitty's best bet is to have that bloodwork done. this will tell the vet where the problem is coming from and just what needs to be done to correct it. 15 isn't too old - with proper treatment for the right conditions, the kitty may have several more years left...but only if you take her in soon. call your vet tomorrow and see what they find out.
as for weird, well, it's a cat. that's their prerogative. :-) good luck with your girl.

Unlike others who have posted, I dont think the cat is dying. It could simply be that she has diabetes. Dont worry, bring her to the vet as fast as you can just to make sure!

Your cat needs to see a vet right away. Unusual thirst is a sign of diabetes, common in older cats. It can be treated.
She cannot be left untreated. Please to the vet ASAP.