Irregular period? pregnant? please help

Sh
2 months in a row my period can earlier then expect in December it came a week early was light then for 2 days stopped completely . This month January I got my period on the 6th but wasn't due for it till the 13th it last 4 days and is completely gone. I have never had this happen to me before I have always been regular. Last unprotected sex was the 4th and before that was November 29th . What does this mean could I be pregnant? What can cause irregular periods all of a sudden I'm not at all stressed I'm actually the opposite of stressed. Please help me figure this out.