wierd period shorter than usual had sex a couple of days before period!!

i had unprotected sex about 4 days before my expected period. i had it a day late started monday i really thought it was gonna be normal but it wasnt i had it for 3days my regular period its usually very heavy and always lasts a whole week. the 4th day i didnt
bleed the whole day till about 7 and it was just for a little while.. the next day the same thing happened i went the whole day without bleeding till about the same time and was just a small amount. my period had never came this way and never lasted just three days! is it possible for me to be pregnant or what can it be? i was told if your period isnt normal somethnigs not right can anybody help????
please!