Is it normal to get vertigo when looking at the sky?

I'm not afraid of the sky or anything, but whenever I look straight up at it I get really disorientated.

I first noticed it when I was 5 or 6. My babysitter made me lay on the ground and look at the clouds. I hated how big the sky looked. All I could think about was how awful it would be if I somehow fell into all that blue. It began to feel like gravity was shifting; that instead of laying down I was standing up with the sky stretched endless in front of me and nothing under my feet. The only thing worse would be if I were dangling over the sky with only the grass I was holding to keep me from falling off the Earth... So naturally I imagined just that and started freaking out. My sitter asked what was wrong after I ran into the house and, being 5/6, all I could say was that it felt like I was upside-down.

I've tried it a few times since then, to see if I may have outgrown it. I still get the same feeling, but only during the day. I love looking up at the sky at night, I even find it relaxing. What's really weird is that I actually love heights. I like being in airplanes and hot air balloons. I even want to learn to fly a helicopter. There's just something about standing/laying with my face to the sky that really freaks me out.

So what do you think? Is that normal? Does it happen to anyone else?