What can I do if my ex is now claiming back child support and I'm already 3 years into a Chapter 13?

The first thing every lawyer learns is that clients don't always tell the truth. So, I don't believe that you've been paying child support in cash and it has left no evidence whatsoever. Also, child support is included in Chapter 13, so your ex should have been listed as a creditor in your bankruptcy. If your ex was listed as a creditor in your bankruptcy and failed to file a proof of claim for back child support, she can't claim it now in a state court action. The issue of whether she was owed back child support on the date you filed bankruptcy has already been decided by the bankruptcy court and she can't get a second hearing in different court. Besides, she is prevented from suing you in a state court by the automatic stay of bankruptcy. If (and I don't believe it for a second), you actually did pay her in cash, there will be evidence. First, she will have either deposited it in her bank or she will have paid expenses with it. If she paid expenses, it will be obvious from examining her bank account, payment cards and credit cards. For example, if she has been buying groceries with the funds, she won't have any payments to supermarkets.

You need a bankruptcy attorney who easily solves these matters.

Most child-support obligations are not dischargeable in bankruptcy. Which means that they stick with you. It's a pity about the cash, since proving such payments, as you know, is difficult. But you can try. For example, if you made the payments soon after cashing a check, or better yet, depositing one, and you can show a consistent pattern, that can help. (Of course, if you were using cash because you were being paid in cash, perhaps to avoid income tax, you took the risk that you would not be able to prove payment.) I wish I had better news for you. But you can almost always work out payment plans with the child support agency: they usually care much more about someone making regular current payments than they are about collecting arrears, so you can usually arrange a monthly payment which you can manage.

Your divorce lawyer should have told you that you never really know someone until you divorce them. You could tell it to the divorce judge and let the court decide who they believe. This is called learning the hard way.