Why do I have to pay back my ex if I never got the money?

My ex has paid for my children since 1998 which is great, however, the csa based it on his wage at that time period.

In 1999 his wage went from 12,000 per annum to 35,000.00 per annum. I wasnt fussed, i was raising my children and unemployed at the time,and not even in receipt of the payments, they went to the csa.

Then in 2010, the csa sent my ex a cheque for £2000.00 for overpayment because of a new law set up in 2006, which meant fathers should not have been paying such a high percentage. The problem is, this is now my debt.

My argument is, why is it my debt, considering a never received those payments in the first place, and why didnt they investigate how much my ex WAS actually earning before handing the cheque over to him. Please someone advise me,i did start working late 2005 and have been working ever since.