From a veterinary technician's point of view, I would tell you not to worry.  UTI's are very common with dogs.  It could be the surgery, but most likely what caused it is that Gracie has been holding her pee longer then normal since the surgery and when dogs hold their pee for too long they can very easily get a UTI.  I've had my own dogs in the past get UTI's post surgery.  Fortunately, it is an easy fix and the blood should be gone within 48 hours of starting the antibiotics.  I wouldn't worry unless Gracie continues to pee blood several days after being on antibiotics. 

thanks so much for your replies! I appreciate the reassurance. I'm very glad to hear mast cell rarely affects bladder/kidneys. I knew that was true for lungs so all added info is helpful.

No, a culture wasn't done, I think so she wouldn't have to be put thru another trip to the office etc. If not improved in 2 days will do actual testing. However, I think she's already not peeing as frequently so will take that as a good sign. She goes outside so it's pretty much impossible for me to check for blood…

Since being on the prednisone she's been drinking a lot (expected w/ the prednisone) & for about a week has been peeing on a seemingly normal schedule. I realize different antibiotics work for different reasons but I guess I was surprised she got the infection after being on the other antibiotic for a full schedule.

thanks again to both of you!