Khilanath,

I think i had exactly the same type of blister some 12 years ago. It came and went for a month or two. When i visited my school's dr. , i beleive he said something similar to what Shirish is saying , "its a blockage of the minor salivary glands" and that it didn't have any cure. However, each time it grew in size, i let the doctor puncture the blister ( yes the blood will come but it won't hurt) and eventually it stopped coming up. Hope it helps.