It might take a little time for your hymen to get out of the way completely. Also, sometimes sex causes cramps, especially if it's the first time or really vigorous. (I also get this weird bruised feeling if the guy's penis hits my cervix a lot.) Depending on what's going on, you might talk to your boyfriend about trying a little different angle, or putitng his penis in more slowly or more gently, and see if that helps with the cramps.