I personally never had that, but my mom has had it for YEARS (like maybe 20 years) and so has my friend (about 5 or 6)! Neither ever went to a doctor about it, but after so many years and it never got worse or turned into anything, I'd say it's anxiety. (they both have anxiety issues) they just say their whole insides are vibrating. My mom said hers is like that all the time, some days more pronounced than others. My friend's comes and goes.