besides the obvious reasons listed above by everyone else, and I skimmed over most, so if someone told you this already, I don’t know.
So when I first got my horse, Atreyu, he was very cinchy. He’d tense up and would sometimes have a bucking fit when it was tightened.(or so I was told by his previous, not so nice owners). The first thing I did with him was wrap a rope around him, right where the cinch would go and would slowly apply pressure. I did this for a week before putting the saddle on him and trying to cinch him up. Granted my horses behavior was because of someone yanking the cinch up super fast and hard and not going slow.
So he associated cinching up with pain. I had to retrain him on almost everything and am still working on things. But cinching up is no problem now.
So this idea might not work for you. Since it sounds like this is something she just started doing. But like someone else said, this could be an old habit she’s starting up again.
I would check all your equipment, make sure there’s nothing that can poke her, etc. and then move down the line. Vet check, see what they say. I personally don’t believe that body work works for all horses(it didn’t do ANYTHING for my horse and he was actually miserable afterwards). But it could work for your horse.
Good luck. I hope you find out what’s going on. Hopefully it’s just her being naughty.
"Take the time it takes, so that it takes less time."
"Expect a lot, accept a little, reward often."