Vaginal bleeding - not bladder stones, likely not UTI.

[Lulu's Mom]

Please note: I AM working with my vet on this, not just trying to get free medical advice

My 1+yr old spayed Jack Russell mix, Lulu, was licking herself and there was blood in her urine. The 1st urinalysis supposedly revealed she had vaginitis, and the Humane Society I was fostering her for put her on an antibiotic (I never rec'd the actual results of the test).

I have since adopted her and started taking her to my own vet. A second urinalysis revealed: high WBC count, very low pH, no blood in urine. He put her on a different antibiotic (the 1st one caused gastro problems).

The bleeding stopped during meds, but resumed <2 days after the last dose. A bladder ultrasound revealed NO stones, no inflammation of the bladder wall. Vaginal cell swabs showed no indication of any stage of estrus (we were afraid that perhaps the spay done by Humane Society was 'incomplete').

The next possibility is that she has a polyp. My vet did not see a polyp in the entrance to her vagina, but did not have an endoscope to examine the full length of her vaginal vault. Before I take her to a reproductive specialist, I thought I'd see what info & suggestions I could find...
(1) An on-going case of UTI/vaginitis seems unlikely to me, since the culture failed to show any bacterial growth. BUT:
(2) If it is NOT a UTI, and is a polyp or something else instead, then why does all the bleeding stop as long as she's taking the antibiotic?

Any replies, helpful suggestions or insight any of you might have from your own knowledge or pet-owning experience would be greatly appreciated! Thanks.