My store - I always rode English and even after months off I'd get back in a dressage saddle and feel totally comfortable. Then I had a bad fall and cracked my sacrum (part of spine near pelvis) and stopped riding for ages. I bought a stock saddle for security but whenever I rode in it I got a backache about 20 minutes after getting off (even when sitting in saddles in stores). I sold the saddle and started riding my friends horse in a western and did not have a problem.
Then I bought a different horse who is currently too narrow for a western so I pulled out my dressage saddle, and after I ride I get bad lower back pain. Did it again today - same result. I think it's the curved seat (rather than the flatter middle bit of the western) and the tilt to the pelvis. I still ride "dressage" in western saddles, but for some reason I am fine.
I am feeling a bit sad - worried I will never be able to ride dressage properly again. And that I'll never get a saddle to fit my mare.
Love to hear your experiences!