Until you can see a dentist, watch what you eat.
When was your last dental checkup and cleaning? Any trauma in the area? Change of toothpaste, mouthwash etc?
It could be an injury, it could be an allergic reaction to something, it could be an infection (buildup of tartar and plaque) or it could be some type of disease such as lichen planus. Its hard to say.
I would recommend seeing a dentist or your primary care physician.