Tax refund was never discussed in our divorce decree...can I amend the decree now?

[kitty 01]

Due to having bad lawyers, our divorce decree doesn't mention the taxes we filed together in 2008 for the 2007 tax year that we were living together and married. We agreed (and I have proof of that through emails) to split the refund of more than $6,000. She spent it all, and told me tough luck. I thought about what I can do to rectify the situation and sent her a letter that stated that since she decided to take my half of the return, I would consider the amount of $3000 a lump payment for child support. I would continue paying child support when that amount was satisfied, in August. I started payments as stated in August and received a letter from the child support agency stating that I owed $1300 in arrears!!! I called and explained that I made a big payment earlier in the year. They called her and she said it never happened. They took my tax refund for the 2008 tax year and sent it to her and are now garnishing my paychecks to pay off the "arrears" that I supposedly owe. The IRS doesn't care about the situation. Apparently it is not fraud to take someones half of a refund. What do I do? She lives in California, I live in Colorado. Do I modify our decree here? Sue her here or in California? Even if I modify the agreement, she has taken the $3000 and is currently collecting more every month. Please help!!!!