Child Support

I live in Utah, child support is taken directly out of my check. I have no problem paying child support. Right now I am on medical leave from work due to surgery. In the past the children lived with me for a 3 month period. My question is am I obligated to pay full amount of child support while on disability, and if not do I have to repay the balance after I am able to go back to work? Also the three month period that I had the children, I paid the full amount of child support, I was told later that I only had to pay half, can the balance be used to pay any that is deliquent due to disability? Another question I have is who determines when and how much child support can be raised? I have never been notified of court hearings but my child support payments have been raised twice and the only notification I receive is a letter from the state telling me that the payment has been increased. What is the formula used to determine a child support payment? Looking forward to hearing from you. Thanks