If you have already called the BFS offset line at 800-304-3107 and they tell you there is no offset under your social; it may be a non-tax debt offset and you can try calling 888-826-3127 to speak with a rep to find out if there may be other non-tax debts you owe that could cause an offset.
The government will also be sending you a letter to let you know exactly what it was for. That letter will have contact information for who it was offset for and you can contact them directly if you disagree with the offset. It generally can take up to 4 weeks to get a letter from the government explaining what happened.