So I think I figured out my problem. Yesterday at the barn, I did what ohmyitschelle said, about rubbing the back and girth line, and it worked! At first, Jubilee didn't like it... but she got used to it after a couple minutes, of me rubbing. I worked my fingers down into her coat, until it was nice and warm. I also walked her around before that too. Then I put on the saddle, and did the girth up one hole on each side. She didn't even flinch!
And her girth was a lot looser too. Usually when I do it up the first time, its quite snug. Then I walked her around for about 10 minutes with the saddle on, just one hole. Then re-tightened it again, and she was fine.
I also did a similar thing with the bridle. First I warmed it up in my hands, as I always do. Then I put it in (took a little coaxing), lead her around in the arena with her tack on, brought her back, took bridle off and let her go for 10 minutes. Then, when I put the bridle back in for a second time, she took it without an issue!
So yes... I definitely think her muscles were just cold and cramped. The walking and rubbing DEFINITELY helped. Thanks so much everyone for all your input!!