What Causes Mid-period Heavy Bleeding With Clots After Sex?

hi i am a 20 year old female i have a question about my menstrual cycle, i havent been sexualy active untill now with my first partner and when we have relations at first he does not use protection for the first 5 or 10 minutes and after that he put the condom on to finish, most of the intercourse is protected. i had my perdiod 2 weeks ago and i got it again 2 days ago, it usualy lasts only about 3 days and is fairly light, this time it was extremely heavy, bright red and today is like a non stop river, i go to the bathroom and a thick clot / substance comes out as i wipe and the heavy bleeding continues, i would like to know why that happened should i be concerned? and im not on any kind of preservatives. please help