I have ear fullness on right ear only, occasional headache on right side around ear, and occasional ringing. What could cause this?

Eustachian Tube Prob. Eustachian tube dysfunction (etd) is probably what is causing your problems and the underlying problem that causes etd is seasonal allergies. Treat your allergies and the etd goes away. Try otc meds and if the problem persists, go see your doctor asap to rule out ear infection, etc.