Do allergies cause a feeling of pressure in one ear?

Yes! Absolutely. The eustachian tube, or the passageway that connects the ear to the back of the nasal passage, can become inflamed or clogged with allergies. This can lead to increased inner ear pressure, either on one or both sides. By relieving the nasal congestion/swelling, typically the ear improves.
Possibly. There is increased secretions in the naso pharynx with allergies as well as swelling of the mucous membranes if severe enough that's the basis for nasal steroids. It is possible to have secretions get into the tube from the ear to the back of the pharynx which can obstruct causing a pressure feeling.