d.a. seeking $ that is not owed

my ex-wife and i divored in 96 we split the kids i moved to los angeles i later got into some trouble with the law and my son was placed in foster care with my mother the co. of los angeles would not allow foster care aid so my mother had to use a.f.d.c. or welfare. i did everything the court had requested of me i cleanned up my act and i have gotten costudy of my son i have paid back all of the child support now los angeles county d.a. is going after my ex we have a divorec decree in the state of washington (yakima county)which states no support for either child can the d.a.overrule this order? also as i stated earlier i paid all support the d.a. is suing my ex for 2,800 backsupport plus current i never requested this so if i paid all the back support how can the d.a. say she owes can i file fraud charges against the d.a. for seeking to collect money that is not owed los angeles county california i have been to the d.a. office many times about this matter they give me the run around and no straight answer i have a court date 01 30 01 please help thanks