could I be pregnant?

Okay so I had unprotected sex and no i'm not on birth control, November 21st, (which I ovulated the 20th because I can feel it drop) and for years now my period has always been 6 to 7 days (i know long) and i just ended it and it was only 4 days long.. it has never been that short in my life... could it just be different hormones that made it shorter?? Or do you think i'm pregnant? I'm starting to get worried because it was soo short..