I spent the next few days having someone hold her and playing around with her, putting my weight across her, in the stirrup, and none of that bothered her. I had someone hold her when I stepped in and climbed on, no issue. She understood my leg aid for a walk and a trot, but not a canter. The vet said she had a pregnancy before, and it caused her to be off balance, so she can't find her balance to canter quite yet. (She's a lot better now! She canters perfectly while free lunging!) I figured we'd take it step by step until she started barrel kicking out at every horse I rode by. I rode for a half hour - hour everyday 5 days a week with another horse in the ring to get her used to the fact that she has to share. Eventually that came to a stop but something else I learned about her was that if you give her her head to relax, she will jump and buck and spin and once that starts, she will not give up twisting her body in every direction to dismount you.
Take your time, work with a trainer, and don't do so much with her all at once!! End on a good note. If you have to do something that provokes a violent or outrageous reaction, break it down for her.
Cantering after a lot of time off is not a "she as pregnant and lacks balance" thing.. it's a stamina, muscle-memory, strength thing. If she doesn't canter under saddle for a long time, she'll be rusty. Baby or no baby, that's the truth.
It sounds like you've gotten far with her already from bucking with weight in the stirrup to now worrying about cantering. Get her all nice at the walk and trot first.
And I wouldn't rule out pain yet.