What legal grounds does a medical company have if you owe them money and the Ins Co. sent you the check?

So 2 years ago I received a $12,000.00 check from my insurance company. I contacted them and they said it was from my surgery a year prior. I inquired on who the money belongs to (spoke to numerous people) and all they would tell me was a company from Fl. I offered to send the check back and they said no, they would just return it. So I deposited the check, held onto it almost a year...things happened, money was spent over this past year. I get a call now from the company that it is owed to (2 years later) and of course they want their money. I offered payment plans but they want it in 3 installments...I don't have it. They are of course throwing out the attorney word. What is the worst that can happen if I don't comply with the amount of money they are willing to accept? A judgement against me, on my credit record? What legal ramifications do they have?