Help?

I've had Nexplanon in for almost 5 months now and this is my first period since. I have been sexually active and my period has now lasted for three weeks now and is still going. My period has never been normal so I am a bit concerned. I have also had ovarian cysts before. What should I do? There isn't much pain which is amazing but the bleeding isn't going away and when it seems to be slowing down it becomes heavy again. I do have some mild cramps.