What Causes Vaginal Bleeding?

my bf came out on my ass and i fear that his "thing" tickled down to my vagina....which i had immediately gone and washed it off...for further protection i had an ipill....now after 6 days my vaginal bleeding has started..i know its the side effect of the pill...could you tell me how long it would last? am expecting my period to start on arnd 20th....will it get affected?