Ear blocks during public speaking

Most of my work involves public speaking. Recently a situation has developed where my left ear blocks up when

projecting my voice to an audience

< this only happens when I am speaking to an audience

< the problem can begin after an hour of public speaking;

sometimes it can kick in a little later; occasionally it doesn’t happen at all

< when it starts, it keeps coming back during public speaking time

< when I stop speaking loudly it goes away

< it’s not painful, just annoying as it muffles the sound of my voice

< only very occasionally I’ve had this problem in the past but now it’s more frequent. I am 63 years old

< I’ve had my ears rinsed by a doctor removing wax blockages but it hasn’t resolved the problem

< I’m trying to figure out what triggers it: swallowing, for example, I drink about half a litre of water (sometimes more) over a four hour public speaking session

< has it something to do with the way I use my voice?

Many thanks.