Behaviour question: Headshaking

[Hexe]

After being stabled for 2 weeks with little exercise due to injury and snow my horses started to shake their heads. It started in the stable and is now mainly after being put a bridle/headcollar on and does not occur while being ridden in a contact only on a long rein, neither on the field. The ridden horse also started swiching her tail when being asked to trot on and on other occassions...
I am mainly worried as especially one of them got fairly suddenly holes over the eyes, which I have heard is a sign for stress or pain and also seems a lot calmer than usual.
The vet said the horse looked good but had a quick check due to the headshaking and couldn't find anything obvious and so we tried soaking the hay as a first step, which did not improve the situation.

Should I worry? Should I get the vet out again? I feel my horse is unhappy, but can't think of a reason why :/

 

[orangepony]

Is your horse turned out again now that the snow has passed? Mine have recently been on box rest for 4 weeks (various untimely injuries) and one (diagnosed headshaker actually!) developed a 'flicky ear' when headcollared. I think he was just narked at being in as he doesnt start proper headshaking until late april, but since having urnout again any traces of the flicky ear headshaking have disappeared again.

It could be linked to the recent temp increases, is she overheating in her rugs?