What Is going on?!

Long story short - my period usually comes on the 16th, last month it came on the 10th. On the 8th this month i had sex and started bleeding a lot during it, there was no pain and so i assumed i was getting my period. But then it didnt come. Its the 11th, im freaking out. Why did i bleed and have no period, and if im pregnant why did i bleed. Help :(