Is this a thing?

So my ex and I have been split up for almost 10 years and have 4 kids together. He has been on child support since 2019 and since then, 2 of the kids have turned 18/graduated high school. The amount of the child support has never changed and he has made comments over the years that he still shouldn’t be paying for the older kids. I told him to file an amendment which he never did. I finally went and filed the paperwork just for a review of everything and now he is saying that he has a top notch lawyer and I’m going to be responsible for paying him back. I’m sure he’s just talking out of his ass but could I be responsible for paying him some sort of “back pay”. We have mediation on the 27th and I’m nervous now.