How Can Menstruation With Chronic Mid Cycle Bleeding Be Treated?

I am 21 years old, married, and have been on birth control for about 8 months. My periods while on birth control have been very consistent with no irregularities, but for the past 3 months I have been getting periods in between my normal periods. The first two months this happened the bleeding was light (not even one pad a day) and brown/black and lasted for about 10 days and would start a little over a week after my last "real" period ended. This month the bleeding, again, started in between my period, but has lasted longer and is heavier and brown/black. There has been a little cramping before the "in between" periods would start, nothing really bad. My "real" periods have not been affected by the in between bleeding as they have been on time and normal in color and quantity. I have never been pregnant. I should add that I have been dealing with depression (excessive sleeping, change in appetite, not leaving the apartment, loss of interest, feeling helpless, crying without provocation)for a few months and I have been feeling very anxious and stressful for the past few months also. What is the cause of this in between bleeding? How can I get my period back to normal?