Depo BC Shot

I just started my Depo shot after giving birth to my son In June. I've had my second dose and this month I've had a three week long period full of bleeding and cramping. Is this normal for a new user of the Depo birth control shot? My cousin said this happened to her when she first started and my mother feels I need to go get checked at the doctor.. but when will it stop and get back to normal?! It's annoying 😩. Opinions? Advice?