Quick sharp pain in rectum?

I sometimes experience a rectum pain that is difficult to pinpoint. it is very quick and sometimes a sharp pain but sometimes a dull pain. i cant determine exactly where in the rectum it is, maybe halfway up? it seems to be more apparent when i have my period. what can it be? anyone else experience it? i'm scared it is cancer? I have had bloodwork 2 times in the past year and has been normal both times. I just had an internal uterine ultrasound, also normal. I have no blood in stools. I am a 32 year old healthy vegetarian female. no medical problems, no drinking, no drugs