I've ben staying with my mother since October 2012 and just ben approved for ssi finally. However, now that my mother knows this she is claiming that I owe her for rent and wants 90% of my backpay. But the thing is.. we never agreed to me paying her. I thought that $2000 was enough money to give her since I ben giving $100 in food stamps and $160 from my a&d each month but she wants so much more

You are being sued by your mother?
You win a 'case' in court.
At home, you sit at the kitchen table with paper and pencils and try to stay calm and write down what she says and what her expenses are from month to month, and then tell her you will meet again tomorrow after you think about it, and try to pay her as much as is right.
If you can't agree, get someone else who you both like equally to arbitrate for you.

PS: SSI isn't 'pay.' It's welfare, taken from the paychecks of working people and given to you. It's OK, it's for your disability, just please call it what it is.

Make sure you have her sign a receipt for what you do give her. Then tell her to take you to court for the rest. Sounds like you will be getting enough that you will be able to move out on your own.