Western saddle fits??!! ?

Ok so if you have read some my other questions lately, my horse has been crow hopping at the lope. Anyway, so she was pretty bad a little while ago(read about it in question: sell my horse or not). Well for Christmas, I got a lunge line and lunge whip and I tried lunging her yesterday. She was really really good! Almost perfect! She didn't crow hop at all. And I then got the idea of lunging her with the tack on. Well right after I tacked her up and started lunging her, she started being bad. At the lope on the lunge with it on, she kept crow hopping and could barely do the same lope as she did before without the tack. So is it my saddle that doesn't fit? I have been thinking it is for the past few weeks now. Right after she was doing the poor lope and crow hopping, I immediately untacked her. She also has a saddle sore right by her withers, people have said its the pad but I think the saddle could be doing it more(and it IS called a SADDLE sore!). Anyway, what do you guys think? My mom thinks its just her attitude but why would she start crow hopping with the tack on, on the lunge? And she never used to do this so....?