Paying a medical bill.

In January 2009, I was hospitalized. My insurance covered all but ten percent, which I would have to pay to the hospital. I received the bill in the mail with the amount of 10% that I owed. I called the hospital to make payment arrangements. I told them I could afford $10 a month, the said no and told me I would have to pay $75 a month or I would go to collections. They took my account to collections, I sent them a letter stating I would pay $15. I sent two checks. They are now they sent my account to a lawyer and are going to sue me the amount. What should I do?