It could be irritation from the UTI as well as you could have developed a yeast infection that is causing irritation. Any itching at all? Otherwise, it sounds hormonal. When we take birth control pills, during the placebo week, we aren't really having a period. We are spotting. This is because the hormones that are manipulating our system prevent us from ovulating and our uterine lining to build up as it would otherwise. So, you spot due to the cessation of the hormone you take with active pills. Does that make sense? That might look a little different than usual this time around but still just the in between bleeding that you have.
Otherwise, you should take a home pregnancy test to rule that out. They are accurate 2.5 weeks after your last sexual encounter. If this continues, see your doctor to discuss! Let me know how it goes!