She could have a bad tooth (as opposed to points). This will create soreness...
Or a loose tooth..
Ditto those possibilities.
In terms of infection, if there's anything "brewing" in her mouth, you can tell by the horrible odor it will start to emit.
Smell her nostrils and smell inside her mouth, as well. If you do that once or twice a week, you will at least be able to stay on top of any infection that might develop.
It doesn't matter her teeth were looked at six weeks ago - with older horses anything can happen a week after the vet or dentist visits them
Two months after the vet gave all my horses physicals (including teeth), my then 24 yr old started quidding his hay really bad.
The molars way in the back had hooks on them and his hay cud would hang up on them. The vet knew they were there but didn't think the hooks were that bad; he didn't want to anesthetize this horse because he also has serious metabolic issues and is very drug sensitive on top of that.
The teeth ended up getting floated anyway but the vet brought out a special type of "Happy Juice" to reduce the reaction.
I said all that to say, it's also possible there's something going on with the molars way in the back that perhaps the dentist didn't see or he did see and was hoping wouldn't get worse, as it can be risky business giving these older horses these kinds of drugs.
I think you posted that beautiful face on another thread
Smoochies on the Snoot to the Grande Elder