Help ? Im trying to get pregnant with baby #2

After I had my baby I didn’t get my period. When I went to my Socorro I told them that and they said it was fine and I got the shot the 3 month one. After that when my baby was 3 months I got it but it wasn’t my normal period it was just spitting and a few small spots of blood where I wouldn’t fill. A pad it was a waste I could literally use one for a day without it even getting blood on it. I had to go again in June for the shot again but I didn’t due to it gettting me really fat.so now when I got my period again It was the same thing but this time is lasted 3 weeks and then i got it again lasting the same amount now i got it this month and it only lasted 2 days and not much of bleeding just wiping and there was blood. We’re trying to start on baby #2 but I’ve read that after the shot some woman can’t get pregnant ?