Canker sores, if that is what they truly are, are usually caused by a virus. Lysine will help with that, and there is a toothpaste that will help with it too. The virus is always there, but becomes more active when the body is stress. My sister gets them on her lip anytime she has stress. There is an ointment for when they are on the lips.
I have been fortunate to not have this problem personally. I do have a cat with the feline version, which is physically expressed as leaky eyes and can advance to a respiratory infection. I add a lysine supplement to her food since we travel and that may increase stress.
But it could also be from biting the inside of the mouth, chewing, etc from being uncomfortable. When I had braces, I would get sores in my mouth from the metal cutting my mouth. You might try using a mouth guard for awhile and see if that helps. You may be grinding your teeth or chewing in your sleep.
I would also see how your mask is fitting and see if it is putting pressure in a bad spot. You may need to find a different mask or a different style of mask.
Who would have thought it would be this challenging to sleep and breathe at the same time?