Was/is there an order in place requiring your ex-gf to pay you child support from jan05 through aug05? If no, then she doesn’t have to pay you anything.
Who is “they” in your case? Are “they” asking you to enter into a consent order - the “agreement” you refer to - in which you agree to forgo your right to seek what appears to be retroactive child support from jan05 through aug05? Ask your attorney why she recommends you not seek back child support for the period you had custody of your son.