Period problems.

So I've been on my period since October. I have been on and off birth control since I was 13 years old, I am now 22. This Sunday (January 24) makes three weeks since I've been back on the pill. I am still bleeding and now I am starting to cramp really bad. Should the birth control have started taking effect by now? Do I need to go back to my dr? Or should I give it more time? Also my dr thought I had pcos (poly cystic ovarian syndrome) but found no cysts when an ultrasound was performed. Sorry this is so much to read. I would just like some feedback. Thank you in advance.