well, first of all, I am NOT saying you have low platlettes. I am basing everything on my own experiences and thats it.
First of all, I would go to the doctor and get a CBC done, to check the platlette level. They should be 150,000 to 300,00 I believe it is. If they are LOW, it will explain the blister(s) in your mouth. If they are low, any good doctor can and will put you on some type of steriod (usually pregnizone), which usually helps build up the platlette level (I know they dont ALWAYS work on people...myself being one of them). Also, he may want to keep a close eye on your platlettes. If they are low, and you start bleeding, your blood will not clot properly. Also, if they are very low, you risk spontanious bleeding as well as bleeding of the brain (the second being an extreme case).
There are many methods doctors try to use, to build the platlettes up to a safe or "normal" range. As mentioned, steriods is one, immuniglobulin is another (IV treatment......I HATED this one!!!). Also, in some cases, Chemo is used and in eextreme cases, removal of the spleen (the spleen produced an antibody which kills off the platlettes in your body, just as fast as they are created).
As mentioned to you earlier, it may just be acid reflux. But, I wouldnt sit around and wait for it to go away just incase it is something serious like low platlettes. I have been fighting this problem since September of last year, when I was hospitalized for three months because of it. So, I speak from experience when I tell you this. I have finally been able to get my platlettes up to a good target zone for me (100,000). That is still low, but it is a high for me!!!!