I Was Nearly 3 Weeks Late For Period I Had

I was nearly 3 weeks late for period I had spotting on and off during that time. I actually started but it was heavy and already been going for 16 days. My gp prescribed norethisterone . Im on my second day taking these now. I stopped bleeding within couple hours of taking the first one but today I started bleeding again. What's going on ? I just want it to stop now I'm sick of bleeding.