If I had sex then got my period the very next day, but it only lasted about 2 1/2 days, could I be?

No... u ovulated before u had sex, the egg from ur overies already attached to ur uterine wall.. Sperm doesnt fertilize the egg after it has already attached itself to ur uterus.Shortly after ur egg attached itself the menstral bleeding takes place, and considering u said u started ur period the next day, ur all set.... :)