You were standing right? Maybe you were standing still for too long with your legs straight. It can cause your knees to lock and blood won't be able to circulate through your body good enough and you will pass out. It happened to me at a kottonmouth kings concert once... I started feeling warm and I knew something was wrong, before I knew it everything was black and I felt this cooling sensation moving up my body(the cooling was my body falling to the ground) my friend picked me up a couple seconds later and I was surrounded by a big group of people just staring at me.

That could be what happened to you.