Could it be from a specialist consult or something? They bill separately from the hospital. And it usually is pricey.

It could also be a hospital billing error. There are so many possibilities (system update, new employee error, system error, and so on.) Unfortunately, the best I can tell you is to contact them directly and find out. But definitely explain the whole issue, that they have tried following up to see about charges since Nov 22 (or whenever) and were told there were no payments due. It may seem repetitive but, unless you're speaking to the same person every time, there's a chance they won't get the whole story. There's only so much room for notes in those programs used.

Good luck, I hope this issue gets resolved in a clear and quick manner!