What's going on??

So I started taking the pill for the first time on the 30th December and that's the day my period started. I was heavy and then I gradually got lighter and for the past 3 days I hadn't bled at all so when I finally decided to not wear a pad today I go to the toilet bad see blood! Although it was very brown. It's not day 13 of my period. Is this because of the birth control or something else? Please help!!