period is late

My period is now 5 days late and I'm starting to get really nervous. I am seeing this guy but I highly doubt that any of his cum has gotten in me but it is a possibility. Is there anything I can do to stimulate getting my period? should I be worried about pregnancy? Why could I be late?
Also, I've been getting this really bad pain in my stomach that doesn't feel like period pain or diarrhea.