Had sex for the first time--now my period's super heavy??

Audrey
The first time I had sex was Friday. It hurt, and about an hour later I noticed a ton of blood in my underwear. Over the next two days, it thinned and eventually stopped.
Then I started my period, like normal, expect that the flow was really heavy. I bled through a tampon, my underwear, and jean shorts in 6 hours, a normal amount of time for me to leave a tampon in.
Has anyone else had this happen to them? Did it go away? Also, why is it happening?