Got a medical bill after 3years

I got a mail saying that I owe approx $4500 to my hospital where my first child was born more than 3years ago. That hospital has transferred that bill to collection agency named FBEC. Inc, who has just sent a mail saying to pay negotiated payment which is $2700. This has come out of no where we had settled everything. Have any of you guys gone through anything similar and what did you do?