Kat, it would seem you have had either an allergic reaction to something, or you have an infection in the area. The sensitivity to light that you describe sounds as if your pupil is dilated and not presently able to contract. Imagine a camera with an aperture (light control) setting which is stuck - it floods the lens with light and washes the colour out. In the same way, your eye is temporarily unable to regulate the amount of light entering which is quite unpleasant. You do need to seek help from an eye doctor, but you will be OK in due course. The swelling indicates an external source of interference (allergy, infection) and not a neurological interference, and this is good news.

If your eyes are often sensitive to light (without the current problem you have) it could indicate a problem with the pupils along the lines that I have described, or a lack of pigment in the iris which can also lead too much light hitting the retina at the back of your eye. Again, and eye doctor will be able to advise.

Don't panic, you'll be OK. Just get an appointment as early as you can.