SSI direct deposit's , claims they have over paid me?

I am a 56 yr old and disabled, both physically and mentally. I share living expenses with my 77 yr. old mother whom lives with me and she only receives her S.S. each month, which is even considered way below poverty level. Even with both combined monies, we are still below poverty. SSI sent me a letter, roughly 2 years ago stating that THEY had made an error in my direct deposits and that they will be taking out each month, of my SSI ( which we all know SSI does not give much money as it is ) until I have paid THEM back the full amount of $3,500. They are taking out, each month, money from my SSI. It's hard enough as it is living on a BELOW poverty level, let alone having to accept the fact that they are taking money out each month. My mother is also physically disabled and has to pay a co-pay for medicine? Really? ( shaking my head) Sickens me to think that this our "good ol'e U.S.A." No one in America should have to go hungry and have to pay for ANY medical expenses when they become seniors. This makes me sick!