Has anything changed recently? Have you recently done a water change? They convulse and shrink right down and back up to flush water out of themselves, and take in new. Most do it through the night, an "internal water change", but if it's been that way for a few days, it seems it doesn't want to take in the new water.
As you know, keep your eye on it's mouth, the tightness of the mouth is a really good indicator of health.