Saddle fit and peeing? (long)

[RhndLev@cs.com]

Hey guys, I've been having some trouble with Special.  About five weeks ago, 
she started getting -- well, weird under saddle.  Grumpy, refusing to go 
forward and then suddenly charging into a canter if I insisted she go 
forward, refusing to gait (she's a TWH).  On the ground, she remained very 
mannerly.  Then, she began peeing excessively, but again, only under saddle.  
She was not and has not been lame at all and was not showing any signs of 
soreness in her back.  We treated her with sulfa antibiotics for a urinary 
tract infection, and she began peeing a little less, but the other symptoms 
remained the same.  

Then, about two and a half weeks ago, the vet found some soreness in her back 
just in front of the point of her hip about two inches on either side of her 
spine.  We tried to get a urine sample, but the mare would not pee in the 
stall (she was there all day and into the evening before I gave up).  But, 
under saddle, she was peeing three to six times within 12 miles.  
Unfortunately, I couldn't convince anyone to go riding with me to catch pee!  
So, we decided not to use a catheter on her (yet), and just treated with a 
more powerful antibiotic.  I had the farrier out two days ago and he looked 
at her and thought the problems stemmed from poor saddle fit pinching a nerve 
in her back.  I had never heard of poor saddle fit causing a horse to pee a 
lot.  He said that he was willing to wager that if I rode Special again after 
almost five weeks off of riding, that she would do her gait again and be much 
more forward, but would rapidly become worse again. 

I rode today for only four miles, and Special peed at the start of the ride 
(within 10 minutes of the saddle going on) and at the end of the ride.  Urine 
is clear and odorless.  However, she was doing her gait for me and was much 
more forward.  

So, has anyone heard of a pinched nerve causing the problems I've described?  
Failure to gait, sure.  Lack of impulsion, sure.  But peeing???

Rhonda