I don't know if she got injured. It very well could be with the scabbing and such. It could be related to the cold, but you've said that she's a bit of a snotty horse, at least that's the impression that I've gotten. Is she running a fever?
I cleaned the site by the knot and got to skin. I couldn't see anything on her inner jaw though. She either had an opening there too or the stuff just drained and dried there. Sounds more like injury and a cold doesn't it?