Two instances this occurred: climbing on top of a building and on top of a mountain- both weren't very tall, maybe 20 blocks max. Both times when I got near the top I started losing health as if a something was attacking me repeatedly, could hear the attack sound, then the water bubbles appeared near my health bar as if I was drowning. It continued for about a minute before it stopped. There was nothing around me and I was on dry land, during the day.