There is something...
...called canker sores. These are sores in the mouth, which are harmless otherwise, but really painful. I get them often. These days I have them too, and it is soooo irritatingly painful.
Most of the articles on the web say that women are more prone to these than men, and also teens and people in their 20s. However, I fall in neither of these groups. Emotional stress is also considered a cause for these. Incidently, I once consulted a dentist, and she also said the same. However, I have a feeling that I get these when I am pretty much relaxed. Strange.
Whatever may the cause of these may be, for about 4-5 days its too much pain in the butt, or for that matter, in your mouth.