Bleeding after sex

I’m not a virgin but tonight I started bleeding after I had sex. And it happened about a half hour after we finished, and it was a lot of blood. I was wearing running shorts that have a lining in them (built in underwear) and it went through the lining and the shorts and on his couch. The couch is okay but I want to know why this all happened. It was the best sex I’ve ever had, but I’m not a virgin and my period ended yesterday and I haven’t bled all day (I’m on the pill and we used a condom too). Any answers? Thank you in advance