Why am I on my period after only being off it for 7 days?

I am 21, I just got off Marina BC in NOV I have only had my period back for about 3 months. But this month I had it and 7 days later it is back. Not just spotting I know what that is.. this is a full blown period like my usual ones... I have had unprotected sex in the past week I dono if that would have something to do with it... if someone could please inform me on where to get more Information or has experienced this it would be awesome to here what you hae to say!