I get them. I get two, always in the same spots. One on inside of lower lip and the other i get in the way back on the roof of my mouth. the one on my mouth comes more often and i've seen a correlation to irritation (such as eating or smoking). I usually pop them with my finger nail.
I never would have thought it was from Crohn's.
(these are different from ulcers that are DEFINITELY from crohn's - ulcers are more similar to canker sores).