
Yes like JG says it could be kidney problems, it really is the most common problem. However a problem is that blood samples will only show up anything once three quarters or more of the kidney function is already gone.(Usually by the time blood samples show a problem, 70 to 80 % of the kidney function is gone.) So you could have a cat with just 50 % kidney function but the blood test would show up as normal. If your vet has an ultrasound scanner, it would be a good idea to scan the kidneys. Also though often the kidneys either shrivel and go small and hard, or enlarge in size (depending on what kidney disease it is) and most vets would be able to feel this, so the very first step would be for a vet to simply check the cat out and feel the kidneys -it's not very difficult.
There is a great relatively new drug called Fortekor which helps increase blood flow to the kidneys and it can improve the quality of life for a cat with kidney problems a lot. It's usually only a small dose of half a small tablet a day (unless the cat is very large) and shows an improvement within about a week. Well worth asking about if a problem is found. An old cat of mine was given another 18 months of life thanks to Fortekor, and it was good quality time, where he felt really well.
Of course, it could also simply be a urinary infection, or a case of the cat not drinking enough for whatever reason, so that the urine is too concentrated and therefore smells more.
Good luck!