I have one now (a few days after a real risky exposure). For me, it's nothing but stress. I get cold / canker sores when I get really stressed out.
No it doesn't. Canker sores are a common condition caused by bacterial infection. My acupuncturist also says that it can be precipitated by stress, or by eating too much spicy foods.
Vitamin C deficiency! Take your vitamins. :)