collection practices in colorado

I live in colorado and have a debt collector getting money from me for a almost 12 year old debt from illinois which was a court ruling with another individual. The other individual has been served papers and is making payments. I was recently served papers and contacted the debt collector and agreed on a payment plan for 3 months. Now I have been sued to have my pay garnished while I am making my payments to the agreed amount. I was told in a signed letter and on the phone that we would reevaluate the payments after the three months but my wages are now going to be garnished in the third month in addition to the agreed amount which was paid. I also have been very confused about the other party that is paying on the debt. I was told by the debt collector that there were payments made but they are not reflected in the total debt they post on their paper work to me. I also have been told by the collector that they don't have any information regarding the other party making payments and that it is up to me to keep track (even though another person at the collection agency did provide the payment information).
I don't know how a debt being collected by 2 parties in different states is regulated?
I don't know if the collection agency can make a payment agreement with me and then sue me and break the agreement they made.