I hope the vet finds what is wrong. Sometimes they get UTI's after surgery but it could also be from discomfort. Very often before surgery vets will express a dogs bladder by squeezing it from the outside. It works well and keeps the bladder out of the way during surgery but the bladder and tummy can be bruised as well. As I'm sure you can imagine a bruised bladder would be very irritating and could even spasm causing involuntary urination. Time will heal a bruised bladder and antibiotics a UTI so hopefully your vet will fix her right up.
Teresa