Debz, I only just noticed you said you're on the injection. Have you been on it for long?
I only ask because I used to be on it and I put on masses of weight while I was (it's the evil meds I refer to on another post I've just made on another thread...). It gave me an enormous and practically insatiable appetite (I was still hungry after eating an enormous 3 course Christmas lunch and enough chocolates to satisfy the biggest chocaholic on the planet).
It sounds like you're not eating that much

but I wonder if it's still doing something to your body which is causing the problem. It is renowned for making people gain weight...