Call your doc, sounds like you may have a bladder infection,not related to Flexeril.
Do you find you are urinating more than usual? Not that is a sure sign, but it would be a good idea to call the doc and have them take a urine sample at the office, you can have it done at your family doctor (GP).
One time,when I had one of the several,bladder infections, I didn't even know it, had no symptoms till I really was feeling lousy, which took longer for treatment (usually antibiotics) to kick in and stop the infection.
I wish you lots of luck with this all.
Sincerely,
DE