What Causes Bleeding Post A Dose Of The Depo Injection?

Hi, i had one dose of the depo injection on 31st january this year, I didn't go for the second dose in april as i have been bleeding constantly since and felt very dizzy/ faint. My doctor gave me Norethisterone around June which stopped the period for 2 weeks and I felt less dizzy, but after 2 weeks the bleeding started again. So in total I have been bleeding for nearly 7 months now. Since then I have been on Noriday and the bleeding is still going, sometimes its light and other times its heavy and i have period pains a lot of the time. I'm starting to feel light headed and very tired. I also had bad stomach ache and diarrhea yesterday... should I carry on with the pill and wait to see if the side effects sudside or try another pill? Or do I need something to stop the bleeding again first?