Firstly im sorry you fell and broke your back, that must be scary and worrying.
I doubt very much that a jolt from the electric fence has turned her into a different horse. If she's always been saintly and gentle girl it sounds like she's got some kind of health issue going on, good for you getting the vet to check her over, the other thing I'm wondering is maybe she just doesn't like the new yard, routine, whatever? Some horses prefer different set ups.
The other thing to bear in mind is she's just moved to a new yard with new horses, new owner, new everything add in to that she will be picking up on your own mental state too, is she getting a good routine? Who's bringing her in and putting her out? Same person every day? Same sort of times? She sounds as stressed by all that's happened as you are to be honest, I wish I knew the answers for you but starting with a physical check up from the vet is a good start.
You also said that up until she bucked you off she was beginning to settle so that in itself is good, I don't mean to sound harsh but it isn't her fault you broke your back, yes she bucked you off, but, you could have just as easily just got up and got back on, the broken bones were an unfortunate consequence and I'm not trying to play that down as I can understand how that must make you feel, but try to imagine how you would be reacting now if you had been able to shrug off the fall? I'm going to guess you wouldn't be half as upset, angry, or scared? I guess what I'm trying to get across is horses, some more than others are brilliant at feeling us out. Sorry I'm not great at putting into words what I'm trying to say, hugs and healing vibes to you and your pony x