Glad to hear that you already have an appt. at the vet.

As far as opinion I'd be guessing: UTI, bladder infection, or stones. If you can get a urine sample for your vet appt. I would HIGHLY recommend you take one. It will make it much easier for them to narrow down what's wrong and how to treat it. Not to mention that in *may* save you some money in doing more extensive tests or meds that aren't as effective that might have been found out from a urine sample.

Hope things go ok. Keep us posted on what the vet finds.