Extreme Jealousy.

There is this guy at my school I've had a little crush on for a few months. It's really not an extreme crush, it's just a little first grade crush, you know? Well, every time I see him speaking, sitting or noticing another girl, I get extremely jealous. I'm not sure why either.
I'm not a jealous person, I never have been before at least.
Like today, my friend Paige was sitting beside him, talking to another guy at their lunch table, and I got that angry, upset type of jealousy. She didn't even speak to him that much.
I don't know why this is happening at all.
Ugh, confusion...