this showed up a week ago and its dry it hurts when i open my mouth is is oral herpes?
it could be a cold sore. buy some abreva, its a little bottle and expensive but its worth it!
also it probably hurts because of the location of the sore. your mouth is always opening from eating and talking which is stretching the skin and probably cracking it. i highly suggest the abreva. it will help the sore heal much much faster!