Not everyone can contract Aphthous Stomatitis (Canker Sores), that is, unless under certain circumstances. Commonly only around 60 or 70 percent of people can have them naturally. These sores can be caused by a number of things including passing them from mouth to mouth, too much spicy, salty, or sweet foods as you've mentioned, or even stress which is most common among people today.
If you're writing is true that after you pick at it, it becomes worse I would see a medical physician about treatments. The US doesn't have efficient items to cure these nasty annoyances though, as in France. In France I had a severe case of Aphthous Stomatitis where it was to the point that it appeared as if I had the Mumps. I couldn't eat because the pain was too severe. The doctor gave me a prescription for some mint pills and after a few days they were gone. Meanwhile treatments here only hide the pain, but don't actually make these things go away. Still, it is your only option.
