Did you have a court order to pay child support, or were you just paying her child support without an order? If you were, yes, you will likely need an attorney, because she could have been collecting welfare and not reporting what you were giving her. As such, you will be ordered to repay welfare. Or, she may have decided to file a retroactive child support order on you.
In such a case, it will be up to the judge to decide whether what you have paid so far is child support or a gift? It is in the state’s best interest to declare it a gift, because of the federal matching funds they get by ordering you to pay the retroactive amount.
What I’m saying here is that your ass is likely grass and she controls the lawn mower. I have seen a lot of cases like this in the last decade.