ex-wife overcharged insurance premiums

In my divorce decree it was agreed that my ex-wife provide the insurance for my daughter and I would pay her the amount for the coverage. In TX the father is responsible for insurance coverage. Since she was remarried and had 2 other children and never provided me any type of insurance policy or receipts showing the cost. I have paid $120 a month and recently saw a copy of my daughters coverage meant for her grandparents. It shows she is covered under her step father's policy with the 2 other children and has been for quite some time (5 yrs). I can only assume her portion of the insurance premium is under $20 and has been.
The decree states I should have been notified of any changes and feel this was intentionally kept from me. So long as the child is covered under the health insurance of "ex-wife", upon notice to "me" of any increase or decrease in health insurance premiums of "ex-wife" attributable to the coverage for the child, the payments shall be modified to the new amount.
This amount is auto drafted from my pay check and I need to know if I can get the over payment (about 5k) reimbursed.

You would have to file a motion asking for a decrease in the amount you pay. You can ask for the back amount, but there is no guarantee the judge is going to grant it. They usually will only back date anything to the day the motion was filed. You may want to consult an attorney