Son and / or girlfriend may have fed some treats, some human food, that she is not used to, and she had an allergic reaction. But I do think this is stress. As Karen said, the older we - er, um, I mean the older kitties! - get, the harder it is to deal with change.

Time will tell, if the antibiotic helps and things clear up, then if it stays gone, you will have a good clue.