Yoshi - Luv the new name btw!
Thanks!! One of my cats is called Yoshi.....
If you can lunge your horse first without the saddle and then with to see if you can see a difference.
The other thing that comes to mind is, that even though your saddle is shorter than that of the previous owner, it could have been their saddle (not yours) which has caused some back pain and that your horse is reacting to your saddle which is now hurting her back, if this makes sense?
Some other factors to consider:
* If a horse is cooped up in a stable with no exercise for a few days, then he will be full of beans due to pent up energy
* High energy foods: look at what you're feeding your horse. Is she on haylege or hay? What's your spring grass like? Do you feed her oats?
* If she is newly broken in, then perhaps she is having trouble accepting the saddle and rider weight. The reaction is to buck to get rid of it.
I would eliminate all these things one by one and then, if the problem persists, try to get an osteopath if you can, to rule out physical pain.
Has she had her teeth checked recently? Teeth can cause all sorts of problems. She might have a wolf tooth which is hurting her due to the bit in her mouth.
Wouldn't it be so much easier if they could talk??!