Can my ex reinstate "excused" or "pardoned" back child support?

Several years ago, my ex wife "excused" the child support that I had owed over the years. I was making payments regularly. They were being directly taken out of my check at the time. It just stopped. She went to the child support office and signed a paper stating that she wishes for the support to stop. I went online to check the balance, and the account had been closed. No balance due. She just threatened to reinstate all of the back child support. Can she do that? I know that she can open a new claim. He has been living with me for the last year or so and just moved back to her house.