Irregular periods postpartum

I am 4 months postpartum via csection. I had my first pp period about 8 weeks after having my baby. I started birth control after 6 weeks. It lasted almost 2 weeks then I bled again about a week later then again for 2 days 2 weeks later. I just had another period a few days ago but was short and only dark brown blood. I called my ob and they said its probably the birth control. I didnt have this issue with my first pregnancy. Is this common? I also have frequent cramping like period cramps without bleeding. The thought of even being pregnant again terrifies me but now I am scared of things like cervical cancer. I am definitely calling for an appointment but has anyone experienced such crazy periods after having a baby and being on birth control.