This can sometimes be caused from eating foods that haven't yet cooled to a safe temperature, but can also be due to fever blisters that form inside the mouth, rather than on the mouth.
http://www.britannica.com/EBchecked/topic/299124/Jacobsons-organ
an organ of chemoreception that is part of the olfactory system of amphibians, reptiles, and mammals, although it does not occur in...