I can think of a few possibilities. If your snake has recently struck and hit something hard, like perhaps the glass of the tank if that is the kind of enclosure you use, it might be a bruise.

Sometimes snakes will rub their noses. I guess it might be possible the damage would show up on the chin depending on where/how they were rubbing.

Just throwing out ideas in case something will make you go "oh, that's it!" Without a pic, it is really hard to even make a good guess.

I would recommend getting a look in her mouth. It isn't hard, you just have to decide that you are going to do it and be gentle yet firm with the snake. There was a description of what to do in the thread you gave the link for.