I have irregular periods and the mirena (levonorgestrel) but i've been on my period over 4 weeks now. ultrasound okay. small functional cyst. what could be causing it?

Breakthrough bleedin: Sometimes (not often) the Mirena (levonorgestrel) can cause irregular, 'breakthrough bleeding' from an effect of it's hormones on the lining of the uterus. Sometimes extra estrogen (in gel form, or in a birth control pill) can be used for a little while (a month or less) to try to stop the bleeding. You should also be checked to make sure you haven't been exposed to any stds.