My baby deer (fawn) has a lump on her throat?

I have had baby/wildlife training but, this is my first fawn. I have been bottle feeding her (as she was an orphan) for about a month, she was given to us at around 3 or 4 days old she's doing fantastic but has developed a lump about the size of a walnut on the bottom right side of her throat... Any ideas of what it could be? And what can be done for it?