You need to find a way to look inside her mouth, make sure that the gums around the affected area isn't turning a different color from the rest of her mouth.
Lots of BTS have lip scales that turn out like that, but this is pretty pronounced.
If she's over a year old, I would not worry about her not eating for a few weeks, that is completely normal, especially when weather turns cooler. Grumpiness can come with it as well.

Breeder and keeper of Meraukes from 2010 to 2022.