She could coincidentally have a urinary tract infection that is causing the blood to appear in the urine. Symptoms of a urinary tract infection include frequent urination, increased thirst, squatting for a long time but not getting much out, accidents in the home. There is a risk that the bladder or the urethra may have been nicked during surgery though and she needs to be evaluated as this is a severe condition. I would take her today to the emergency center to have her urine checked. Hopefully, she just has a bladder infection that can be treated with antibiotics. I wouldn't recommend waiting until tomorrow.
It could depending on the bacteria that is present. But I would really highly recommend a visit to the vet to rule out injury.