vaginal pain & tightness?

within the last couple months i'we started taking birth control pills... my doctor mentioned all the regular side effects, which i already knew about thanks to my older sister. i noticed my increased sexual drive/want to have have. but recently i got my last period a week or so early... after that sexual intercorse has hurt a little bit, only when starting of though, around the tip of of the inside of my vaginal hole, and i've also realized it's been tighter. and it also hurts the most when im onto during intercorse in doggy style position it doesn't hurt at all, when im on bottom it only hurts a little?! i was wondering if it may be a side effect of the birthcontroll, or whatnot? what do you think?