What could it be if your eyes swell up and turns red whenever you wear your contacts?

You may be over-sensitive to having something in your eye, for instance because your cornea is not perfectly smooth, or you may be over-sensitive or allergic to the protein buildup that happens on contact lenses, or your eyes may not like the restriction of oxygen that contacts cause.

There are thousands of possibilities, and the kind of contacts can make all the difference too, for instance there are special contacts that allow more oxygen through.