What Causes Longer Duration Periods After Sex?

After having sex for the first time, my periods have been significantly longer. Instead of 3 days, it's lasted about 5 days and 3 days of spotting for the past 2 months. I started having sex regularly about 2 months ago and i do have sex during my period. I also have been on the pill for the past 7 months. After I started on the pill, my periods were short and right on time until sex came. Should I worry about this? Why is it happening?