I get a clear blister every now and then also, but not like you...I think it can be caused by anxiety. Since anxiety causes many stomach disturbances (with acids and things like that) your saliva may be off and therefore causing this...I am not a doctor, and have no idea if I am explaining it right, but I hope you understand what I am trying to say...hang in there. I am sure that it is nothing.
I really think it's related to stress; physically, it's viral, I believe, but is being caused by the stress and anxiety.
Hope this info helps. Best..........
My young daughter gets these in her mouth from time to time. I think they are called mucocele and they are harmless. You can google them and look at pictures of them online to see if that is maybe what you have. Good luck.