No record of loan!

School says that 2 years ago, I was overpaid in student aid.I requested debt validation and they sent me a list of charges. One of the charges listed was for Direct unsub and sub. loan. Neither one of these charges are shown on the national student loan data systems for students nor on the loan service site (nelnet). I asked where the charges come from and I have yet to receive a solid answer. As a matter of fact, I have received 3 different answers. I pay any debts that I come to owe, but I refuse to pay anything that I do not owe. I added up the charges and they do not match what is show in the nslds system.