Why do I have to pay child support if I am no longer a parent with rights to my children?

I gave up my parental rights, in order to save my children from being placed into foster care. My mother is now their legal guardian. I was sentenced to prison and just recently was released, just got a job and my paycheck was garnished 60%. I did what I thought was best for my children, but my mother has been receiving all kinds of services and support from different types of agencies.