You need to look at your court papers to see what the amount of child support that was ordered. If it is more than is being witheld you may be liable for arrearages. You can file an Order to Show Cause with the court that heard your original case requesting a modification of child support if you can not afford to pay what the court has ordered. The court will consider any changes in your circumstances warranting lowering payments. It sounds as if maybe in your case child support was ordered without any income and expense statement from you. If this is the case, your best bet is probably to go back to court for a modification.
There is a maximum percentage of each paycheck, both net and gross, which can be withheld for child support. If the child support order was based on an income figure which included substantial commission, bonus, or overtime in the prior year, the amount that is withheld from a base wage paycheck may be less than the total order.
To calculate arrearage, you simply multiply subtract the amount actually paid on the current order each month from the amount of the order; add up the short-fall, if any, for each month, and add 10% simple interest per year to any shortfall starting with the month in which payment is missed or underpaid.
Be careful in crediting payments once there's an arrearage; under some circumstances, once there's an arrearage, the law dictates that payments are applied to due interest forst, and only when all due interest is paid, are payments applied to principal. That means you pay down principal arrears LAST.
You probably should review the order, and your payment history, with an attorney.