Perhaps the only thing that will put your mind at ease is to take a pregnancy test (Campus Health Pharmacy sells them for $4.99). While stimulating each other to orgasm with your hands is very low risk for pregnancy (with intercourse obviously being the highest risk) the key is to make sure your boyfriend ejaculates away from your vagina. As long as the semen doesn’t get near your vulva (the folds covering your vagina) the chance of pregnancy is almost zero.