I have a 5 year old mare.  Had been in training for a year, came home for the winter and turned out.  I rode her often while with the trainer.  Mostly turned out for the winter.  I rode her in March, no problems.  I again get her ready to ride in April and she is suddenly terrible to put the bridle over her ears.  I turned her back out thinking maybe she bumped her head and it was sore or something.  Accepts the bit fine, is fine when I take the bridle off, but this is not her normal.  So the questions is:  What could cause her to suddenly become head shy?  Dental, ear infection, mites, somthing in the ear???  I need to take her to the vet to see if he has any insight, but wanted opinions here first.  

Have her ears checked. She could have ticks, does she let you rub her ears at all? The only time I ever had a problem with ears it was because of ticks. My horses will let me check them, sometimes the ear will get swollen at the base and thats a sign of ticks.  

Edited to add: When you take the bridle off have you accidentally bumped her teeth anytime? 

I tried to feel inside but cant wrestle her enough to look inside.  That could be possible, even though we dont get many ticks, infact I havent seen any at all this year.  

It seems like here we go threw a period when ticks get bad and then no problems with them for a few years and then bingo we got ticks again. Its like every 3 years the ticks get bad and then poof they are gone. 

Your best bet is just take her to your Vet and have them deal with her. Is she shy about halters too? 

Have your Vet check her poll area too maybe shes out and that making her sore. Have you try palpating her poll to see if maybe she sore there?  

Yes, but not as bad, which is why I go to teeth.  

I know personally where I live the flies are something fierce this spring and my poor horses have had crustiest and sore ears this year! Maybe have a vet look at them and start trying to protect them. I started using Swat and purchased the fly masks with covered ears.

Palpate her poll and see if maybe shes sore there. 

I kinda thinking it could be her TMJ maybe?  

Aural plaque will ruin them,its awful.

Back in the day when I was hauling heavy and could afford it I used a veterinarian that practiced eastern medicine.  I had a horse that became head shy and I couldn't get a bridle over its ears.  We couldn't find anything directly ear related.  She said the ears of a horse were tell all signs of other things going on with the horse.  Something about the meridian pathways in the ear.  Anyway she did acupuncture and showed me where to use acupressure and it resovled the issue.

Something to consider if a traditional diagnosis and medicine doesn't resolve your issue.