spectrum:A tight mask could be the culprit. How long did your mask impression last on your face?
This was my first thought. I've had this problem several times.
It begins by tightening your mask too tight because you don't want water leaking in. Then as you get deeper and deeper, equalizing the mask can't relieve the pressure because the strap is too tight.
You don't notice the problem at 30 feet because the pressure is a lot less. But at 50, 60, 80 feet, the pressure keeps ramping up. 30 feet is only twice the pressure of the surface. 80 feet is nearly 4 times the pressure.