underpayment ssi for minor son, overpayment for me?

my son has been disabled since birth. i was disabled since 2002.
i was divorced in 2001. so we were getting alimony and childsupport. i got my ssi in 2004. my alimony was up this may 2009.
i call to see what my ssi and sons ssi would go up to i was informed ON the phone son had a underpayment of $5,919.00. great BUT i had a overpayment of $4,990.00. SSi blamed childsupport/trustee i went there spent hours while the trustee figured out everything. amounts did not add up. SSI estimated on there part. Should NEVER have been a mistake. i dont work. only support from ex. I NEVER HAVE got ANYPAPER work from Social Secuity showing me why. just that a fed worker i have to ask her to use any of the money in my minor sons saving account and SHE will ok or deny it. He gets his reg monthly check(which went up sence i no longer get alimony). i get a check that went up but they aree taking 10% out every month for what she for ever.
She will not send any papers. i hired my own lawyer to help me.
but why do i have to ask her perrmission to use sons money for the things he wants or needs? i have never misused any money.
please help if you can
thank you