Why is it so much if I’ve never been assessed?

Hi in September last year I received a letter saying I owed the csa 135 .00 then in November I received a letter saying I owed them nothing. This April I got one saying I owed them 4375.00!

Now I’ve never been assessed I made a financial settlement in court with a judge and signed over 50% of my house to her so that was worth about 75,000 since then I’ve only had one phone call from them in February 2012 asking if I was the father.

I told them yes I also told them about the house I give my money to the kids into their bank accounts because I told them I would not pay her a penny because she is dating a serious sex offender and rapist who just got out of jail and who is on the sex offenders register for 7.5 years.