divorce was finalized this week, i have a few questions...

i'm new and i'd like to have someone help me w/ the following. here's some background, i was married 9 years, separated 5, i filed for divorce last year (in CA), 2 minor kids, i paid child support monthly on time(ex and I established). i pay the medical/dental and i've had a girlfriend for the past 2 years. here's my questions: both my ex and i had one child each from previous relationships (her daughter lived w/ us, my son lived w/ his mother). my divorce was finalized this week and my ex/her lawyer slapped me w/ a $30K bill for paying child support to my son why we were married. somebody help me wrap my mind around this. how can the courts give my ex money (which we both earned, she was a stay at home mom, i worked outside the home) during our marriage for payment of child support? why would my ex be entitled to reimbursement for child support payments that i was making for my child? i didn't ask, nor did i receive payments for her daughter living with us. any ideas, regulations/rules, etc.? thanks