Sex stops my period flow

I don't understand this. I have a normal heavy flow, it's day 3 of my period. My boyfriend and I decide to have sex and I take my tampon out, we have sex and when we're done there's no blood. There's no blood on him at all or anywhere else until I go pee. Then my flow goes back to normal. Does this happen to anyone else? The first
Time this happened he asked me if I wanted to have sex on my period, he laid out a towel on the bed and there was just no blood. It's so weird, it's like my body knows I'm having sex lol