What are my rights when sued without knowledge of it?

I received a letter in the mail 2 days ago with what is titled Motion and Certification for Default Judgment". It is signed by attorneys and a judge. It appears that I have unknowingly been sued by a collection agency over a medical bill. This is the first correspondence. I have ever had from them. I knew about the bill. I was a month late in paying and already agreed payment plan with the doctor. They sent it to a collection agency and the agency sued me for the money. However, I have now found out they sent the court documents to my old address and the young lady who bought my old house signed for the papers on my behalf without my knowing it and without informing me. I have now according to this document been sued successfully by the collection agency. The bill which was already high has now got almost $400 of legal fees and is accruing 12% interest - it also says payment must be made by February 20. I am only just finding out about all this right now. This letter was forwarded to me from my old address. What are my legal options here? Should I pay the full amount as soon as I am able? Is this legal that I was sued without my knowledge and someone I do not know accepted on my behalf without informing me? What would you advise I do. I am looking for the most cost efficient and straight forward answer.