My ER doctor (I SWEAR it was John Malkovich researching a part for a movie) was very emphatic in stressing that these things are harmless and that there's nothing to worry about. He said no one knows what causes them, but I am suspicious of caffeine in my own case. I know that the few other times I've had these "floppy fish" feelings it's been when I'm drinking too much coffee. and when I thought back on the day, I remember that I sort of scolded myself earlier in the day for drinking too much coffee...
