Are mouth ulcers more likely when I'm stressed or have weakened immunity?

Yes, that is right. The lining of the mouth is sensitive to your immune state, so when fatigue, stress, and lack of sleep build up, canker sores tend to appear more often and grow larger. Enough sleep, protein and vitamins in your diet, and cutting back on irritating foods all help prevent them. However, if they recur several times a month or several appear at once, an underlying medical condition may need to be ruled out, so please come in for an examination.