What Causes Vaginal Bleeding Every Time While Having Sex?

Hello. I have been on depo provera for almost half a year.Im 20 years old. I dont know if this has anything to do with my depo but for some reason i believe it does of the fact iv never had these symptoms. For the last month eveytime i have sex and climax i bleed. Sometimes its a lot, sometimes its not. Since i have been on depo i have no period at all. So this is the only time I bleed. What can this be from? Also to mention everytime I clean myself up, afterwards i put a tampon in just incase it is my period. But when i take my tampon out there is no blood at all.