Regular BM's, dull headache, no appetite. Relief comes next day after very dark stool (green or gray)

I have a regular bowel movement. Every morning after coffee: stool is normal amount and normal color. Every few weeks I wake up and notice a dull ache in the back of my head and neck. I always think that it's tension in my neck muscles but the dull headache persists and I realize as the day goes on that I have no appetite and it feels like nothing has moved through my digestive system. It feels like it just sits there rotting in my intestine, giving me a headache. Usually the next morning I have a bowel movement and it's always very dark. It's hard to tell whether it's dark green or gray, but it's not brown. After that movement, I get my appetite back and my headache's gone and I'm fine. Sometimes the recovery is more gradual, taking two days. I would greatly appreciate some feedback. This is confusing because I'm fine in the interim between episodes. Every time there's been no change in eating preceding it.