Mare pins ears

[SunsetOvrC@aol.com]

My mare pins her ears whenever I ask her to trot or canter, not only when I use leg aids, but also if I only use a voice cue.  She seems to do it more in the arena than when on trail.  I have just had her teeth floated and changed to a saddle that fits her well.  Her back is no longer sore when I palpate it.  So what is it?  A bad attitude, or memories of pain?  How can I encourage her to stop it?  

Pamela