Why won't me period stop?! Please help!?

I have been on my period for over a month now. About 2 weeks ago it did stop. But then like three days later i came back on. I have had the contraception injection and i dont know if it is that. Last week i had an ultrasound scan to see if there was anything to see, but the lady said everything looked normal. What could this be please help!!! All my periods last forever! I'm anemic due to it as well!