Saddle/Back problem

[BZVU47A@prodigy.com ( DENISE JOHNSON)]

Hello ridecampers.  I have a back problem, on a 4 year old mare, that 
I hope someone out there may have seen before.  She develops a small 
swelling or bump just behind her withers, adjacent to her spine on 
the left side.  It reminds me that the saddle is pinching her causing 
the swelling.  It's about the size of a quarter.  I've tried 3 
different saddles with different trees yielding  the same result.   
The saddles are OF cutback, Boz, and a Fallis western saddle.  The 
pads are a coolback used with the Boz, and a thick pad used with the 
Fallis. She was started last Nov.  I have been riding her lightly 
until late July and August when I began riding her daily.  The 
problem started in mid-August.  After about 1 mile of riding at a 
walk she starts twitching at her withers, switching her tail, 
occasionally looking at her sides, bumping my feet with her nose, and 
now shakes a couple times while out on the trail.  When I get off and 
walk she appears comfortable.  When I unsaddle her I find this same 
bump each time.  It disappears after 24 hrs.  There are no visable 
wounds.  I just gave her 2 wks off and rode her today with the same 
problem occuring.  This is my first back problem on a horse and I'm 
very puzzled.  I tend to ride with a loose cinch and a cowboy told me 
today while out on the trail to cinch her really tight and see if 
that would help.  It didn't.  She's a Al-Marah Seadream filly and I 
was hoping to get her ready for an LD soon.  BTW my other horses 
haven't had any back problems and all do well with the above saddles. 
 Any help would be greatly appreciated.

Thanks.

Denise