Unnotified bill, then collection notice from Credit Management Company

I realized that the company named Credit Management L.P. is in collection of the unpaid balance from Comcast Cable.

I moved last year from my previous address to my new address now, and I kept using the Comcast Internet service.

I have never been notified of this remaining balance even though I keep using the Comcast service.  I do not know what this bill is for.  

Due to this collection activity, my credit score was badly impacted.  If it was notified to me properly, it should have been paid in time.  I need to know the way to fix this problem in my credit score.