Occasionally both my eyes swell almost shut and i have what looks like a water blister on my eyeball what is this my eyes really only itch briefly?

Allergic reaction: This is most likely an allergic reaction to something in your environment. Pets, pollen, mold, soaps or cosmetics are the most common offenders. Avoiding contact with them helps if possible. Try a non-prescription antihistamine eye drop, an oral antistamine like benadryl, (diphenhydramine) and cool compresses on your eyes when the problem occurs. If not better see your doctor.