It seems my head
I can deal with an entire airline flight, up until the point of descent. At that point, my ears decide it's time to close up, and it feels like they're going to explode at any moment. My brain shuts off the yawn function of my jaw, and I get the sensation that the blood inside my skull is a few seconds from bursting through my eardrums.
Everyone says "Chew gum! It helps!" Perhaps it helps for them, but I vigorously chewed up an entire pack of gum during the plane's descent, and it didn't do a damn thing. The pain always has me on the verge of tears, and everyone wonders why I have such a problem.
It's not my fault, my head just can't handle the pressure of a controlled descent. Don't ask me why, because I don't know.