Hello,
Is his beard dark at all, recently?
You mentioned that this cropped up very quickly? Who handled him, you or someone new?
Could he have been injured somehow?
I would normally say it appears to be an abscess or a cyst & needs to be aspirated to determine what it could be. You can give it a week or so to see whether or not the swelling is going to go down.
It is possible that it could be a sting of some sort. If you look closely, can you see anything that could indicate some type of sting or bite? A spider bite is a possibility as well.
I do not think it could be stress related, no.
Does he have a good appetite today? Is he having any trouble breathing, or exhibiting any other strange behavior that could be indicative of a bite or a sting?
Tracie