why do I bleed when having sex

My boyfriend and I have been having sex for well over half a year. Just recently I had my period. And it only last 3 days. It was short but I didn't complain. Well later that night we had sex and my period reappeared. So I don't bleed all day until we are having sex. Then i bleed on him. This has been going on for four days in a row. It's not painful when we have sex or anything. It's odd. Does anyone know what's causing it or going on?