Help!!!

So I have a fairly irregular period and I haven’t officially seen it in a couple months. When I say that I mean I’ve had some blood here or there but nothing that lasted more than a day or was heavy at all (which is weird because when I do have my period it’s fairly heavy and lasts a couple of days). However I have been having sex (No I’m not pregnant). A couple times recently when I’ve had sex I bled a little bit, after the fact though. It’s been unprotected but it’s been with the same person for the past 4 months. Neither of us have had a history of an sti or std. What do you guys think is going on? And has it happened to you guys?