What Is The Irregular And Prolonged Brown Bleeding After Taking Contraceptive Pill?

I recently started the contraceptive pill (I'm on my second month) two weeks after my regular period I got a second period but it was much lighter and very brown (almost like old blood) but I have had it two weeks now and it's starting really be a pain. It's not very heavy and can be almost not there some days. It's effecting sex and makes me feel gross. Oh and I am 17 (: