What Causes Vaginal Bleeding After Intercourse?

Hi,

I used to have my period at the end of every month but it all of the sudden changed to the middle of the month and this past month i got it at the biggining. This month i got my period for five days but the last day i decided to have sex. Its been a week that i finished my period and i had sexual intercourse yesterday, i started bleeding again and i have cramps. what is happening? This had never happened before.