head tossing

I have noticed this since I got Snapple, but never really understood why she does it.
Well, Snapple tosses her head a lot, like, even out in the pasture. She never does it when I am leading her, but when I lunge or ride her she does, mostly when I ask her to canter. She does it out in the pasture too, even when she's just walking around, she'll throw her head.
What does this mean? Could she be in pain or something?

It sounds logical that she would be tossing her head in pain, but that may not be the case.

I rode a horse last week that threw his head A LOT, but that was only because he was ticked off that I was making him walk/trot and he didnt want to. He's the type of horse that wanted to just sit in the pasture and graze all day long. Could care less for human interaction, and or riding.

Hope you figure something out!

Peak into his ears and make sure he doesnt have mites. (you could even take a piece of gauze and wipe the inner ear out if he'll let you, and then you can see if you pull any gunk out. Also, does he have any drainage from his nose? Like maybe allergies? It could also be flies bugging him... I'm leaning towards a physical issue becuase you said he does it in the field also....