The eustachian tube is the long, thin connection between your inner ear and the back of your throat. As your inner ear is lined with mucous secreting cells like the rest of your mouth and upper aerodigestive tract, that mucous has to drain somewhere, and so it drains via this tube. It also serves the function of equalizing the pressure between your inner ear and the outside world. Any differences in the pressure between the two can be very uncomfortable, and can distort sound and even make you feel dizzy. Because it often takes a second for this equalization to occur, we all have had the sensation of feeling discomfort when we change altitude quickly until our ears can equalize again.
Because the tube is so thin, it is prone to dysfunction or spasm that then impedes both the draining function as well as the equalization function. Generally, this recovers quickly, but the spasms/dysfunction can sometimes continue, especially when there are other causes of inflammation in that part of your body, such as when you have a cold or any other upper respiratory tract infection. There are over the counter anti-histamines that sometimes offer some relief, and you can speak with your doctor
if the problem persists.