I've been wondering what this was:
I've been on a number of retreats where the primary focus was to cultivate the brahma viharas.
I went through a phase of a year or two when, at retreats (although not in my daily practice) I would consistently develop a really hot head during sits and walks. My cheeks would become very flushed (confirmable by a mirror) and hot to the touch. It felt like heat was coming off my head/all my focus was going up through the crown of my head.
At the time, I was concentrating on concentration, as it were, and wasn't really interested in this--I simply observed that this phenomenon would occur, but would never stop and persistently investigate its Three Characteristics.
It went away, but I am curious--what was it? Because it's gone I can no longer investigate it. I am thinking perhaps it was an artifact of 3 Characteristics. Maybe it was my version of arm-flapping.