I was told by my ex-girlfriend, ( my son's mother), that I am paying $8 a week too much and have been for awhile. When I started paying child support, the judge put in arrearage of $8 for a certain amount. I can't remember what the amount was but I know it has been paid for quite awhile. How do I go about getting the money that I have overpaid?

Go to the office of the Friend of the Court. Bring proof of payments, and bring the court order for child support. If you stop payment on the $8 you could end up in arrears which equals jail... which is not good. Do yourself a big favor and contact the court. Don't believe anything your ex tells you. Remember, believe nothing you hear and only half of what you see.

Signed, the product of the Friend of the Court.

Many states do not have "friends of the court" so that is more state specific. Do you pay directly to the mother or do you pay to the court ?
What does your actual child support order say, get it out and read what the judge ordered