Just a thought, when you take her to the vet, you may want to take a urine sample along. Having just gone through kidney stones with Abby this spring, those symptoms point that direction a little aslo. At 1st I thought Abby was peing normally, until I tried to get a sample & realized she was not going when she squatted. She was doing more od a sit & "grind" than a sit & scoot. Her belly felt tight because she was filling up...she also had swollen anal glands at the same time, bit the vet said the sit & grind is a symptom of the kidney stone trying to pass, she'd instinctively sit & push down to try & stop what she was feeling. She was eating & drinking normally, altho maybe a little more water than usual.
And to get a sample??lol well...I always scooted a tupperware type lid under her when she would squat!