Unusual Problem

["G & J Thom" <thomlabs@netbistro.com>]

My endurance mare from last year started to have
this unusual problem last year after about 700 
training and endurance miles.  On the way home from
a ride she would stop stand uncomfortably and then 
go on.  If she tried to trot it was worse and her head 
would go way up plus her tail is swishing badly.  She
would also bit at her sides once in awhile.  When I
dismount she is a bit better.  I have had her back checked
and have tried different saddles but still this year she
is doing it again.  She likes to eat still and it doesn't appear
to be her stomach but am stumped as to what could be
the problem or what to even ask my vet to check.

It worries me enough that I end up walking her home
as she seems so uncomfortable.

Thanks
Julia