What Causes Excruciating Pain In My Head During My Bowel Movement?

Within seconds of sitting on the toilet with diarrhea this morning, the back of my head exploded in pain. Both sides, from the base of the skull throbbing out to the sides. The pain equals migraines I've had in the past (I'd call it an 8 on a scale of 1-10), and lasts, with decreasing intensity, for about 30 minutes. This also happened last night, and twice in the past two years.