Hi, dont know what causes it.......I got one when I was very young.........probably around 10 years old. I remember that it got pretty large and I had to go to the dentist to remove........what the did was suck it all out. Dont know what it was........all I know is that it was pretty painful. Glad its gone.
My daughter (15 yrs.) had one of those this past summer, she thought it quite funny and was sad when it popped and went away. We never found out what it was from, but seemed to be nothing to worry about.
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Thanks just for being there! Susie
Sounds like you have what is called a "ranula". Just checked "search" and it is there if you need more info about it.I also had one many many years ago.Mine kept coming back after it popped and finally I had to have a physician tend to it. No big deal though.
Sounds to me like it could also be a clogged salivary duct. I had one a few years ago. It was very painful, and it looked basically like I had a pimple under my tongue. The whole area swelled up, I saw an oral surgeon, and he said that if it keeps coming back he will remove it. So far it has not come back. My dentist recommended sucking on really sour candies to make myself salivate more and force out whatever was clogging it. (sometimes a piece of food can get stuck). That worked. One day I noticed a little tiny hole where the "bubble" had been. Apparently whatever it was went away on its own. I sucked on lemon drops for a few days, and wrinsed with salt water to prevent infection. It came back once after that, and I did the same thing and it went away and has not come back since, knock wood.
Bump/bubble under tongue 
