What causes a small painful bump under your tongue in the salivary gland?

** This is a post from a medical forum, this is not my personal response! **

"A bump under the tongue that comes and goes sounds like it may be a mucocele or mucus extravasation phenomenon. In this location, it can also be called a ranula. Essentially, a mucocele occurs when a salivary (spit) gland under the skin develops a leak in its tube or duct that guides the saliva onto the surface of the mouth. The spilled saliva collects into a small pool that produces a soft bump or lump and this bump may appear slightly bluish in color. The body tries to absorb this spill, but the damaged duct often continues to leak, leading to a waxing and waning course. Minor surgery is often needed to remove the saliva pool and the damaged salivary gland. This treatment is usually provided by an oral surgeon.?