Your body is most likely getting used to the new hormones, or the new pattern of the hormones. I bled for an entire year straight when first getting on implanon (birthcontrol implant) . It took my body exactly a year to get used to the hormones, and now my periods are more or less regulated. If you contact the birthcontrol people or your dr. they will more than likely tell you its normal, but its better to be safe than sorry.
It is not normal to bleed for over 4 weeks. I suggest that you see your gynecologist for an examination and possible hormone blood test, ultrasound and biopsy depending on your age. They can also give you medication that can control the bleeding if the workup is all normal.