You might not be able to do anything about it. If the settlement agreement provided that the settlement you received was payment in full of all claims and/or that you could not sue or otherwise bring any additional claims, then you would be bound by the terms of the settlement--which is an enforceable contract--and be unable to seek compensation or reimbursement for this bill. On the other hand, if their was no settlement agreement precluding you from seeking further compensation if other costs were later discovered, you could go to the driver and/or his/her insurer and seek the additional amount, including possibly by bringing a legal action, if necessary.