VA Garnishment

The Veterans Administration is garnishing my SSDI, but I cannot determine why and how much I owe. I used to be employed at the VA, but that was 12 years ago. I contacted the fiscal office and payroll and was told it was too long ago to be looked up. I got nowhere with SSA, they think it's the IRS, but it's not. The letter I received was from the VA, but the phone number listed is for veterans, not civilians. This is the third time in five years that they have garnished my SSDI. Any suggestions?