Earlier this year I was awarded primary custody of my 2 kids and did not pay support. I was told there is a case law about credit or arrears stating I do not owe support?

Case law in what state or country? There is case law for a custodial parent not having to pay but if there is an order in place then it must be followed until it is modified. Is there an order in place already?

Child support and the laws that it are based on are location specific.

If you had past due child support before gaining custody, you would still owe the past due support, gaining custody may stop your current money you owe, but does not do away with past due in any area that I am aware of.

Also, depending on income, even having custody does not mean you may not owe support ( believe it or not) if one parent makes so much more than the other, and if the other has liberal visit days, some areas figure what each parent would owe based on their incomes and takes one from the other. So there are cases where the parent with primary custody will pay min support if the other parent has very low income and a lot of visits

First, ANY question on law needs to include your general locale as laws vary by area. Second, when you were awarded custody was your support order also modified? Finally, did you have arrears at the time you were awarded custody? Is someone trying to collect those arrears?