Low Period Flow, No Pregnancy Symptoms, Negative Pregnancy Test. Had Periods After Unprotected Sex. Cause?

I had unprotected sex exactly 4 months ago. Since then i have gotten every period on time. This period cam on time also, Today is the third day, but there is absolutely no blood on the pad. Just few drops, Only when i wipe, i usually bleed heavy on my periods. Sometimes when i wipe there is bright red blood and sometimes dark looking blood like when its my last day on period when its getting over. i took many many pregnancy tests and they all showed negative. and i also have no pregnancy symptoms at all. i m really worried what it could be.