You eustachian tube connects your middle ear to your larynx to allow for pressure equalization in the ear so that the eardrum is not pushed in one direction or the other too much. If you hold your nose, close your mouth, and "blow" you are pushing air thru the eustachian tube into the middle ear and pressurizing it. If you've gone scuba diving, you know this is a necessary skill to learn.
I used to get a feeling during and for a short time after really hard workouts that I could hear and feel every breath inside my ear, particularly the left. During efforts in which your breathing passages are opeining up, the inner opening of your eustachian tube can also open up. This is what was happening to me - it became a wide open tube at times when it was necessary that my breathing passages be exanded as much as possible. The long run explanation may come from the fact that, during extended efforts, and as blood glucose begins to drop, the body starts producing more epinephrine and circulating epi increases. This is a signal for brochodilation - as well as the possible side effect of eustacian tube opening dilation.
This is what could be happening to you, but maybe not to that extent if the ear is actually pressurizing. Is it painful at all. An over-pressurized middle ear can be quite painful if pressurized too much. Or is it that you can hear your breath inside your ear?