I guess you haven't confirmed with the department yet. The procedure should be like this: you accept the department's offer, then they will send to the graduate school to complete everything else and send you a formal (official) letter, then you will have to confirm with the graduate school again. You may have to send them some documents. I have done this three times with three programs.
So, if you have accepted the department's offer and the graduate school still haven't received anything, then you have to contact the department immediately.
Good luck!