Braces Billing Question

[Min Yu]

I had braces put on my son last year. There was a fee agreement made. I paid it in full. I paid $600 and insurance paid $1750. When my son about to get his braces off they told me I owe another $4300 . I haven't had no new fee agreement made and they asked why I haven't made a payment since last year February. I told them I was paid in full. They said they would get back to me. They sent a bill to my insurance for $4300 and they didn't cover anything else. Should there have been a new fee agreement made? I was never told about any extra charges. Am I legally obligated to pay them the $4300?