This happens when the root of a tooth becomes infected and fills up with bacteria. It damages the nerves and the pulp tissue inside the tooth itself, if kept untreated, so get to the dentist right away if you think that you might have this condition. A deep crack, fracture, or cavity can lead to an infection.
The most serious root infections cause patients to develop painful blisters. These appear in the form of very sore facial swellings. However, blisters usually only occur once a root infection has progressed to a more serious state. If you deal with a painful or cracked tooth quickly, this should not happen.