I *personally* don't care for that particular saddle... I could never find my seat well in it on the flat or over fences.
That being said, take a look at the way you are jumping. If you are very tight, always having to hold a horse back, or jumping with a roach back, you may be causing yourself the pain. Like others said, a strong core will help lessen the work load on your back.
If you're in serious pain, a Chiropractor or Dr. may be your only answer.
Corporal- your comment regarding another saddle was unnecessary... not everyone can just go buy another saddle, especially with the economy.
If your horse says no, you either asked the wrong question or asked the question wrong
And God took a handful of southerly wind, blew His breath over it and created the horse