Hello, Marsh Chap.  Google "protalgia fugax" or "levantor ani syndrome" and see if the symptoms match yours. At least you might be able to put a name to it if you see your doctor.

One possibility is called “spasmodic pouch.” If that’s what’s going on it’s sometimes treated with an antispasmodic like Bentyl or Levsin. These can have nuisance side effects, but might also help. This doesn’t sound like an anal fissure, even though those can cause spasms, since you’re not reporting the typical intense pain that usually goes along with a fissure. Good luck!