My child supprt was lowered on dec. 15 but dhs says that I still had to make the 372 dollars for the whole mon?

when my child support was lowered on Dec. 15th, i was to pay 372, but it was lowered to 187.50, DHS says that even though the ordered was signed on DEC 15th I still had to pay the 372 till they received the order. they received it in the following Jan. they say that I still have to pay the 372 for both months. Is this correct?