During orgasm, usually the sensations start at the clitoris and spread all over the genital organs from vulva and within the vagina. The vaginal, abdominal and in fact all the body muscles start contracting. Contraction of the vaginal and abdominal muscles might have given you the feeling of coming of menses. You could have rather harsh movements during the sex. The second orgasm women get is a vaginal orgasm. (Very few women have experienced it.) This comes some times after the clitoral orgasm which starts about 1 inch inside the vaginal front wall and then spreads all over again. If you have reached vaginal orgasm, you may get such feeling. Stress also can give rise to vague pain or muscular pain. I do not think there is any problem since after some time, you were absolutely pain free. Enjoy your sex without any stress and you will not get any such pain.