I'd like to 4th or 5th the vet suggestion.
Cassie had a UTI a month or so ago. We took her to the vet b/c one night we noticed that she was pacing by the door & then squatting over & over outside, so it really looked like a UTI to me. Anyway, though we hadn't noticed blood on her, there was blood in her urine when it was tested. Your pup may not be acting different yet, but blood in that area from a spayed girl doesn't sound at all normal to me and it could be a UTI or a number of other things that will need vet care.
Also, if you can't get her to the vet until Monday, if it's a UTI (big if), you could give her a little relief with cranberry pills until then, but I would definitely still get her checked out ASAP! I have a special sympathy for anyone suffering from UTI's. I had a double kidney infection a few years ago, nooooo fun! Please let us know what you find out, ok?