After 5 days of my montly period i had sex with my partner and i bleed, even a day after? what does it cause?

Post coital bleed: There can be a variety of causes from a friable cervix to a cervical polyp. It can also be an STD or other cervical infection. Pregnancy and increased lining thickness also a cause. Most likely to only way to know for sure is to see your GYN a day that it happens, so your physician can see what the source is. Use a condom for this because semen smells like infection