What could be the cause of eye pain/swelling/tearing?

See an eye doctor. Pain and swelling are hallmarks of inflammation. It is difficult to differentiate inflammation from eye infections (which are potentially blinding). See an eye doctor immediately. The longer you wait, the more difficult it will be to treat.
Lots of things! The diagnosis is broad: infection, glaucoma, iritis, abrasion, foreign body, etc. Be sure to get it checked out by your ophthalmologist quickly especially if you are experiencing changes or loss of vision.