If you have a case of the collywobbles, then chances are you should stay home and close to the bathroom. Collywobbles is severe gastroenteritis that may send your lunch on a rocky round trip through your digestive system.
Many common illnesses have the look of a plural but the behavior of a singular (mumps, measles, hives) and collywobbles follows the pattern. The OED describes the origin of the word as a 'fantastic formation' of colic and wobble. That would be fantastic not in the "incredibly wonderful" sense but in the "based on fantasy" sense.