Period and the birth control.

I got my IUD taken out on july 20th and was told to wait until my next period to come to start the pill. I knew i had a while before i would get my period so we did anal sex and oral sex. I got what i thought was my period on monday the 1st, 5 days before my scheduled period and started taking the pill. Weird thing is my periods usually last about a week, Even when i was on the IUD i got my period regularly. But for some reason i only bled on monday. Not even spotting or anything the next morning. Ever happen to anyone else? Or should i be concerned?