Reoccurring Blister In The Soft Tissue Of Tonsil. Had Dry Mouth. What It Can Be?

Question : I have a small sore/blister that keeps coming back in the same spot in the back of my mouth to the left of the tonsil in the soft tissue area between wisdom tooth and tonsil (not between tooth and cheek). You can’t really see it unless I stick out my tongue and then it appears just above and behind the bottom wisdom tooth on left side. After popping it, the area is slightly sore and then the blister starts to grow over the next 24-48 hours when I can pop it with my tongue or finger (max size may be 2 MM, like size of a pimple). after popping it. I saw a dentist last week and he popped it last with an instrument & removed white skin and it came back two days later. I have also popped it 4or 5 times in the last month and it keeps coming back. You can’t really see it unless I stick out my tongue and then it appears just above and behind the bottom wisdom tooth on left side. After popping it, the area is slightly sore and then the blister starts to grow over the next 24-48 hours when I can pop it with my tongue or finger (max size may be 2 MM, like size of a pimple). Any guesses as to what this could be? I've also had a very dry mouth for the last month and wake up 2 or 3 times a night to drink water. No health problems. Run 30 miles per week. Eat health. Ran 7 miles yesterday in 41:49 at Falmouth Road Race so I'm physically fit.