Head shaking... but only in the field?!? Any Advice?

[ez2011]

Hi All,

At the start of this summer, my mare started to head shake for the first time... but it only happens when she is out in the field even though she wears a fly mask. It stops as soon as you put her head collar on to bring her in which I thought was strange (she doesn't do it when in her stable, standing on the yard or when being ridden). She isnt stressy at all, so I know it isn't that.

Any advice would be gratefully received.

Thanks.

 

[Fii]

It may be she has developed a habit because of fly trouble, even though she is wearing a net now.
I was watching one of my geldings last year and he was nodding his head even though he had no fly's on him at the time.