The common cold develops from a virus that leads to inflammation of the membranes that line our throat and nose. The Eustachian tube connects the ears to the nose and throat, helping us maintain pressure in our ears so we can hear correctly.
However, when we have a cold, the immune system sends signals to the body to produce more mucus to help flush out the invading virus or bacteria while triggering an inflammatory response to help isolate and destroy the foreign substance.
The combination of excess mucus and inflammation, although a natural defense mechanism, causes blockage in the Eustachian tube, leading to a feeling of fullness (i.e. clogging) or pressure in the ears.
Such blockage can even result in temporary hearing loss.