What Causes Bleeding After Sex?

Hi I am 24 years old I have had one child about 3 years ago... I have had a few partners and never had bleeding.... I was recently with my bf for 2 years and never had any problems but then i had sex with another guy and i had a small amount of blood but did not start period or irratated... me and my boyfriend got back together and i didnt have the symptoms so i ignored it but recently i had sex with the same guy before that made me bleed and I only have slight bleeding with him??? I am on the Mirina and have no other symptoms