weird discharge

my period this month was only three days late. but when it started I bled through a tampon in about an hour or so... it was extremely heavy. but after it stopped I had sex (three days ago) and since then I randomly start bleeding light pinkish blood for about an hour every now and then and then I will stop. . this has been happening for about three days now and it sends a pain up my body when I sit down... any clue what's going on? (you can't see it well but it's kind of brownish now. it was light pink)