Why do i bleed in :\?

i havnt been a virgin for about 4 years, so i dont understand why ive started bleeding, in september last year i broke up with a lad called michael, when we had i didnt always bleed but i sometimes bled, before micheal i was in a ten month relasionship and a six month relasionship, after michael i ended up getting with a lad called jonny, i bled with him everytime we had , and i hated having with him because it was painfull after, in my past relasionships this has never ever happend, its only started recentley but i know i dont have anything because before christmas i got checked out for every sti, and im usaully carefull when it comes so condoms. but ive bled with and without then :( does anyone have any ideas? if you do it wud be great, thanks :):) xx