Have you ever loved someone that it actually hurts?

It's called neuronal upregulation. Your mind has gotten high off of being around this other person, because they're different and beautiful, usually. You built up a tolerance, like with drugs, but now you're going into withdrawal, and there aren't enough pleasant feelings to make it okay. I used to get it with terrible pains in my neck and back with one girl, until I saw her again. Another time I got the whole chest beating thing around a girl until she wasn't around, then I just wanted to die. It'll get better.