Edit the eye would start watering not watching. Sorry
That is an unusual complaint. If it was both eyes systemic allergy might be expected (e.g. hayfever). But just one eye more problematic. Usually if one eye it is contact allergy, something the eyelids come in contact with generally by rubbing the eye. There are some very rare problems that can cause one eye to swell. You need to see an Eye MD ophthalmologist. Take some pictures of what the eye looks like when swollen to show the Eye MD. If you live in large metro area and have access to Eye MD subspecialists look for ophthalmologist that specializes in "cornea and external diseases".