DCS Inc DCS /Student Loan Corp Livermore, California

I received a letter today from DCS Collections saying that I owed 15000.00 to Student Loan Corp AKA Citibank. saying that I haven't been paying this account. I stopped receiving statements from Citibank in May and thus awaited my next statement from them, which in October when I called them to complain that I had'nt received any bill fro the account , they said that I should have contancted them earlier. I thus agreed to start paying the account again which I have been sending them payments every 2 weeks to bring this account up to day.  Which Citibank has been cashing these checks for the loans specified on the check. Then today I receive this, for which I have faxed them a dispute letter  and copies of my cancelled checks and will call them to complain Monday. I am quite willing to go into court with them and fight it out in there