What do I do if I believe that there has been fraud committed on my child support case

What do I do if I feel that fraud has been committed by the plaintiff on my child support case I am the defendant. My case started in June, 1987. I only came to this realization of the possiblity of fraud when I accross three child support notice for the same day, the same year and in three different counties along with three different amounts owing? I believe that there are a total of 4 different counties in California, and 3 different counties in the state of Iowa and is trying to open a case up in the state of Nevada. My monthly child support obligations have been paid as of 06/30/2003. What I am still paying for is my arrears which now I am realizing that there is no way that I owe.