The cause of your problem is the same as that of the ordinary swollen eyes. Eye allergies are the most common cause of swollen eyes. When your eyes are allergic to something, the immune system overreacts, which leads to eye allergy. You can ask your doctor for prescription medications to relieve the allergy. Eye styes and chalazion can also be the cause. They are the result of inflammation and blockage in any of eye glands. Pink eye may be the cause of swollen eyes. Besides, Cellulitis, Graves Disease and Herpes Infection are also common causes of swollen eyes. The other eye is OK because it is not affected by the factors mentioned above.