Charging Interest on loan in collections

I co-signed for a student loan for my son. I have since gone bankrupt - chapter 7 - bankruptcy has been discharged. The loan was not in the bankruptcy. The day after my bankruptcy discharge they turned this loan over to a collection company. The collection company is charging interest. (How do I find out what interest rate they are allowed to charge or if they are allowed to charge interest) I have made payments as well as tried to come to an agreement for a lump sum settlement. They will not accept what I have offered. It is now with a 3rd collection company. I am dizzy trying to figure this out. The 3rd company stated the 2nd company reported I did not make any payments. I have all documentation of the payments I have made. I was told I had the right to send a letter to the collection company requesting I wanted to deal with the Investor directly - Is this possible? Seems to me I need an Attorney, however I can't afford one. I am physically and mentally exhausted dealing with the phone calls and letters. Thank for any help or direction.