10 day period?

I started taking birth control around a month ago on the first day of my period. Before I started taking the pill, my cycle was about 21 days long and my period lasted for about 7 to 8 days.. My period only lasted about 4 days once I began taking it... I was about to finish the first pack, but even before I got to the placebo pills, I started my period again and it lasted for 10 days; is this normal..? I'm 17 by the way and I got on birth control to stop having so much bleeding