Early period!!

My husband and I had unprotected sex three time while I was ovulating a week and a half to two weeks ago. Yesterday I was cramping and Spotting but today it's heavy enough I need a tampon and I'm still cramping. My period isn't supposed to start until January 5th and it's usually pretty regular maybe a day or two late but nothing over that and it's never early. Can anyone tell me what could be going on?