Have you ever tried the experiment where a delay is added to your mouse input? Just a small delay (a fraction of a second) will feel like something is broken. Your brain will quickly adjust, and before long, everything seems normal. But for a little while, your brain will tell you "there is something wrong, this is broken."
Testosterone has two interesting effects which I think contribute the the feeling of pain after rejection. (Remember that women also have some T, and it works identically.) The first is that it improves muscle function. And the other is that T decreases dramatically when a person is rejected. So suddenly, I think all of your muscles suddenly don't work quite as well as they did a moment ago. So a really crappy physical feeling is compounded by your brain complaining that your entire body is slightly broken because it doesn't react in the expected way.
Testosterone has two interesting effects which I think contribute the the feeling of pain after rejection. (Remember that women also have some T, and it works identically.) The first is that it improves muscle function. And the other is that T decreases dramatically when a person is rejected. So suddenly, I think all of your muscles suddenly don't work quite as well as they did a moment ago. So a really crappy physical feeling is compounded by your brain complaining that your entire body is slightly broken because it doesn't react in the expected way.
Just an idea :)