I'm sure you probably did this, but if you usually use a numnah, did you put one on the new saddle? Years ago, I saw an Arab go over backwards whilst being tacked up at a show(Oh goody! I thought, as I was going to be the ride judge for the Arabs!) An Araby friend of mine asked the owners if he'd ever worn the saddle without a numnah before. The owners said he hadn't. My friend said that could explain it! They put the numnah on, the horse was then fine. In the class, he gave me the most amazing ride (would have loved to have taken him home!) and won the class!
Firelight - her old comfy saddle is an Ideal and the one she took a dislike to is a newer, wider Ideal. How bizarre? Oh well, I guess she isn't unique in wanting to keep her old saddle then. Must be like an old comfy pair of shoes for us I imagine
Arabesque - something to look in to re the muscles behind the shoulder. I think I'll get her routinely checked by a physio as Firelight did with her boy too.
Heidi - yes, every time the new saddle was on her back it was with a numnah underneath.
Thanks again for all of your replies. I'm just so happy that I'm back in the saddle as I was so baffled as to what was up with her.