Is it normal to have this strange feeling?

It's this emotion/sensation thingo that's in the stomach area, roughly where abouts the solar plexus is. I don't know how to describe it, except that it feels really good. It's sudden, like surprise. It is accompanied by shivers up and down my spine, which is similar to the feeling of excitement.

But the strange thing is, the stuff that triggers it. And that is usually seeing something suffer or in pain. Usually by something I find cruel, violent etc. But these things don't always trigger this emotion. I don't know what will and what won't. Some examples are when a villain in a movie kills his underlings, especially if they beg for their lives before. Or when giving a painful injection to a rat (at work, don't worry I don't go giving random injections to rats!) and the rat squeaked in a whimpering way or even just squashing an insect when I was younger. If it's a movie, watching a particular 'trigger' scene too much makes it no longer a trigger.

I was getting it the other day as I learnt to do cervical dislocations on mice. It doesn't make sense, why the hell would that feel good? And what exactly is it? IIN?