I have milky discharge coming from my nipples, why ?
I am on the contraceptive injection so don't think its pregnancy.
Why am I very tired and low in mood?
Irregular periosds too

Contraception injection is basically a hormone shot right? All of the things you described can be attributed to hormones, which you can trace back to the shot. Are you having unprotected sex? Even with the shot, you could still get pregnant. You wouldn't be the first so don't have sex without a condom as well.

The shot is of course not 100 percent effective, while slim chance there is a chance. Without a proper medical exam it is not possible to know the cause,

Have you done a pregnancy test just to see ?