Hello, Bactrime is not known to cause the vaginal bleeding so very likely there is a different reason for this. It is important that you see your doctor and have a gynecological exam to make sure of the cause. This is not urgent but important. Having a pregnancy test meanwhile would be helpful to rule out pregnancy as the cause. You have to also be cautious to distinguish if you are having vaginal bleeding or blood with urine as bloody urine can happen with urinary infection itself. If you bleed in your underwear not related to urination then very likely it is vaginal bleeding. If you notice blood in urine then very likely it is due to the urinary infection. If your urinary symptoms are not improving and you have been taking the antibiotic for at least 3 days then you might need to talk to your doctor about this and have a urinalysis and urine culture to make sure you identify the right antibiotic you are sensitive to and that will treat the infection. Meanwhile make sure you drink plenty of fluids. Hope I have answered your query. Let me know if I can assist you further. Regards, Dr. Antoneta Zotaj General & Family Physician