One of my Eyes Appears to Be Sinking Back into the Socket

Since I was little I have always had large, prominent eyes. However, for the past several years (I'm 29 now) only one of my eyes is prominent. Most days the other seems swollen upon waking and remains that way throughout the day. It isn't actual swelling; this is just the one term I can think of to describe it. The eye is small, as though the eyeball has sunk farther back into the eye socket than the other eye. The eyelid disappears. The result is a drastic lack of symmetry. What could fix this?