The most common causes of blisters in the mouth are cold sores and canker sores. Cold sores are due to Herpes Virus infections and are contagious. They can be brought on by stress or illness. Canker sores are usually secondary to viral illnesses such as a flu virus or cold. Canker sores . . . → Read More: I am a nineteen year old female. I have one blister in my mouth on the right anterior pillar. It hurts to swallow. What could be the cause of this? I have no other symptoms and I have never had this happen before. I woke up two days ago with pain in my throat. I assumed it was a sore throat but I noticed that it hurt near the back of my mouth instead of the actual throat. I have not done anything treat the pain except eat a tablespoon of honey before going to sleep.