Injury or Attitude?? (kinda long)

["Julia Thom" <thomlabs@netbistro.com>]

Last year my 9 yr seasoned arab mare seemed to have
hurt or strained a muscle in her right rear leg.  I had the 
chiropractor out and her back was out so she fixed her up
and I let her rest for 8 weeks, on recheck by the chiropractor
everything was fine.  The lameness seemed to move from
one back leg to the other but was most obvious in the right.
I also had the vet out and he did a thorough check of the mare
and could find nothing wrong.  (by the time he came the 
lameness was not too obvious)

I gave the mare the winter off.  First ride this spring went well
and I thought all was well.  2nd ride today, the mare felt like the
right rear leg gives out and she feels like she is slipping and
she did this 3 times and then she starts acting up quite badly.
Head ducking, pulling going sideways actually a bit scary to 
ride.  She settle for a bit and then start again.  I weigh only
146 including tack so I am not too heavy for her.

I guess I am a bit frustrated as she was a really nice mare to
ride and it sure appears that she is in pain when I ride her.

Any ideas would be much appreciated.
Thanks in advance Julia