That will be the issue. Your ex-wife's motion should set out the reasons she believes you to be in contempt. You can then demonstrate to the Court that you remained in compliance.
I suspect it may be an issue of how much was actually paid, and whether they believe you paid off all of the arrears, in which case it will be an issue of an accounting of the payments made and the amounts owed.
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Are you sure the decree doesn't say pay until 18th birthday or until the child graduates from school? Perhaps that might be the issue.
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