did I have a miscarriage?

so long story short I had unprotected sex and did plan b for the second time in 2 weeks.. a few days later I was put on birth control but my period was about a week late and I thought maybe it's just because my body is adjusting to the birth control. I started getting worried until I started bleeding but now I've been bleeding for almost 3 weeks!! It's very strange for me and my back has been killing me and I've been very nasious as well. I'm not sure what is going on any ideas??