What you are descibing sounds like a mucocele, which is essentailly a cloged, very small salivary duct in your mouth. The entire lining of your mouth is covered with them. When they clog up, they form a little clear blister which is painless and eventually bursts, healing in just a few days. Do a search in the message boards under the term mucocele and you will find a bunch of posts related to them.