Depo provera Injection. Advice please

Hiya! I started the depo provera injection I’d say in December? I’m 16, and decided to switch from the pill because I kept forgetting to take it and I have extremely heavy and painful periods. So far I’ve had 2 injections, but the last 3 months I’ve had non stop breakthrough bleeding. It’s not a lot, but every day I do find I have brown discharge, and sometimes quite painful cramps? I was wondering if this is normal? As I haven’t had a proper period yet, only this brown discharge. I’m starting to get a little worried. Any help will be appreciated! X