Swelling and pain in one eye could be due to many reasons. One important thing to differentiate is the site of the swelling - whether it is the eyelid or the white surface of the globe. If the problem is in the lids, suspect something behind the globe, for example changes due to increased orbital pressure like in thyroid disease or orbital masses or inflammation. If it is in the eye (globe) it could be inflammation of the layers of the eye. In both cases you need a thorough eye examination of the eyes to rule out underlying diseases.