Period 2 days early!!!

So we had unprotected sex a week ago and again two days ago but he didn't cum in me he pulled out, my period was suppose to come in 2 days but it started today and I was having slight cramps and lower back pain 4 days before and I went to my doctor two days ago as well and the pregnancy result was negative. Can sex make your period come early or is this a regular period or something else ...