What Causes Onset Of Menstruation Post Intercourse?

I had intercourse yesterday night... it wasnt a full penetration as it was paining, since i was a virgin/or i am still i dont know coz i dont know whether the hymen broke or no... well now my concern is that i started getting my period from today whereas it was supposed to start after 10 days. What could be the reason?? even it kinda paining in my stomach... Can you help me in letting me know whats exactly happening???