Do you feel horrible? Well, if you felt wretched enough to cry, you probably look horrible, too. Here is why tears, especially emotional tears, come along with puffy red eyes and a puffy red face.
Eyeball redness comes from two different sources. First of all, your eyes don't have permanent bags of fluid hidden just behind the tear ducts, ready to deploy on the off chance that you're flipping through the channels on your television and happen to see that scene in Serenity where Wash dies. The fluid has to come from somewhere, and it's piped in through the blood vessels around your eyes. They dilate with fluid, and suddenly become much, much more noticeable.
Secondly, you'll notice that if you really get a good cry going, your breath isn't coming as regularly as it usually does. This not only makes you sound stupid, it makes you look stupid as well. When you hold your breath, your face gets red. You're slowing blood flow to the lungs. This puts pressure on the venous side of your circulatory system, especially the side that's pumping the blood back to your heart. The blood vessels dilate, and your face and eyes get red.
So just when you feel most terrible, you can always look in the mirror and feel terrible-r. Or you can just assume you look as bad as you feel and avoid the mirror altogether.