I received $5200 back pay today. I called the local SS office and they said it was from SSI. I'm not getting SSI, I'm getting SSDI. I didn't want to argue with the woman on the phone so I just let it go. My back pay from SSDI should be about 12K. I don't know what's going on but I don't want to make waves with the local SS office. Has this ever happened to anyone? By the way, on my SS account on their website says I'm getting SSDI $975 a month and it also says I'm on SSI. I think something is messed up.