What Causes Vaginal Bleeding After Having Intercourse?

Hello, i just recently started having sex (about a month and a half ago). The first time, it hurt a lot, but i didn't lose blood, neither did i the next couple of times, but last week, we did it again, and afterwards i went to the toilet and there was a little bit of blood (a few drops). We waited till yesterday to have sex again, but again there was blood. Any idea what's wrong? thanks in advance! (i'm 18, and we use condoms)