I had blocked tear duct surgery in 2015. my eye always once or twice a year end up with swelling below eye and itchy outer corner that makes my eyelid swell. why is this?

Eyelid swelling: Sounds allergic,but you should see your ophthalmologist to make the diagnosis.

Blockage Persists?: If the swelling is toward the nose at the location of the lacrimal sac and the same eye, there may still be a partial blockage. A famous doctor, Sir William Osler said... No matter how hard you cram and squeeze, make it all one disease. Given your history of tear duct surgery, it is likely related.