Just a few questions
She was in the same pasture as her yearling and he plays too rough
yearling, do you mean you started her at a year and a half, then bred her at 2, and the yearling is her colt(gelding I hope)?
I didn't start riding her when I did because my vet okay-ed it. That's how I wanted to train her. I'm sorry if no one likes it. That's what I prefer doing. Just because they're being started young doesn't mean they won't be able to be sound.
So you started her really young because you felt like it, not because she was ready or your vet approved?
Starting very young has been linked to future soundness issues. The last bones to "close" are those in a horses back, which in a saddle horse are the most vunerable to the stress of weight carrying. Riding a horse that is a year and a half, the knees arn't even closed yet, never mind the back, which is years away from being mature.
I think it is probably a combination of two things causing her to not like her tack. She could very well be sore. As with humans, back problems often don't just go away on their own, so the one time issue could still be causing pain.
Also its fairly likely that she is on the lazy side and thinks she can get away with it.