What Does Slight Vaginal Bleeding After Sex Indicate?

I had unprotected sex with my bf on Friday evening.. It was my first time ..I didn t take any type of contraceptive pill yet. I felt pain but I didn t bleed. but on Sunday evening I saw light bleeding from my vagina . I want to know what could be the reason? Am I pregnant?? however my last period was on 5 March & we had sex on 20 march.! please tell me what is this type of bleeding. !???