Often times, with cats, the first sign that there might be a problem is changes in eating/drinking habits. Cats who begin to drink excessive water and are always found around the water bowl can indicate that there might be something going on inside of her. A few things that can cause this can include early onset of kidney disease, diabetes, or some other problem. I think, at 15, it may be time to do some more diagnostics rather just write this off. I would start with a general health profile (blood work) and a thyroid test/urinalysis. This could give you TONS of information and hopefully it will come back completely normal. At least then we could be more apt to say this may just be a behavioral thing more than something more serious going on. The sooner you do it, the greater chance, if there is a problem, that you can really treat it.
I hope that helps!
Glad to hear the bloodwork was great. The urinalysis actually give us some really good information. With the blood, that often indicates there is inflammation of the bladder. Cats get a condition very similar to interstitial cystitis in humans. Often times we have to prescribe them anti-inflammatory medications to help with that. Often times animals will try to drink more water or hang around the water bowl in an effort to try to dilute out the urine and relieve some of the inflammation. I would talk to you vet about maybe trying some NSAID's. I bet those will help her a lot.