You should contact the child support collection agency and make them aware of the situation. It's possible that you owed more than the payroll company paid out to child support, or that you have been billed twice. You should also contact the payroll company to verify that the check they wrote to child support was cashed, and the problem was really taken care of. After all, they are the ones that caused this situation in the first place.
Know exact amounts when you are talking to the child support collection agency, and have paperwork to back up what you are telling them if necessary (i.e. pay stubs, a copy of the check paid by payroll, etc.) If you have over-paid, eventually you should either be reimbursed or the extra amount will be paid toward your future child support payments. Either way unfortunately, clearing this matter up will probably be an exercise in endurance on your part.
I'm sorry you're dealing with this and best of luck.
Please note that my answer in no way establishes a lawyer/client relationship but is only meant to offer general advice. You should still consult with a local attorney regarding your legal matter.