Sally did you just go to the dentist? Just a question for my brain to think about.

I had a similar problem. Red sores in the back of my mouth, two on the sides, it hurt to swallow. One pimple on my nose and one on my chin. I am WAY over the age to get pimples, so this struck me as strange. None on the lip.

My PCP and Neuro didn't have a clue. My ENT didn't either. Maybe something going around. I had the diarrhea after the fact also. I do not know if any of this is connected to each other or a really close similarity to yours, but I think it is bacterial. Not an Anti-B bacterial type. IMO

What I did:
I brush my teeth three times a day. I starting using GLY-OXIDE 1/2 fl oz., as an oral rinse. I swooshed a bit (one bottle squeeze) around my whole mouth, for a few minutes. It is gentle. It has a mild peroxide (to debride) in it, mixed with a mild wound healer. You spit it out and rinse once with a tiny bit of water so not to swallow the stuff.

When I went to bed and in the morning, I put a dab on my finger and put it on the pimple on my nose and chin. BTW, they were a bit itchy I don't know why. But they all went away.

Whether they would have gone away on their own, or not, I don't know. I saw improvement within one or two days inside and out the mouth.
I hope you find something that helps or buy this OTC stuff in the toothpaste isle.
Lady