If she is on top of you and you're getting ready to ejaculate into the condom, you're suppose to make her get off you first then take off the condom.
the chances are slim.. don't worry..
Hi, nothing much to worry, hence ur GF is on birth control moreover the reproduction process starts only if the semen gets into the vagina and the sperm into cervix, so its noteable that ur GF will not get impregnated. Am not a doctor though but i believe my suggestion will help :)
But am unable to find answers for my queries :)
God bless~!