What Causes A Prolonged Menstrual Period?

Hello,

I am a female 23 yrs old. I have very infrequent period schedule. My periods only show up once in 3-4 months. This time my period has started and has been going for over 3 weeks now. Granted, I observed that it tends to show up when I stop taking adderall. I'm still taking adderall right now and it wont stop. I have had 1 abortion before and never had kids. My period has never been "normal" since I've had it. I am not on any birthcontrol right now, was on Nuvaring when I was about 17 & stopped it b/c it messed with my hormone/mood level too much. I don't take any drugs other than adderall. A friend suggested it could be a miscrriage but I just took 2 pregnancy test which came up negative. What should I do? What could be the reason for this?