Every time I would touch her hind quarters (no pressure, just like a finger touch) she would really "hump" her back, sucking her hind quarters under in a pretty extreme way. There were no marks or visible swelling. Her left hind seemed to be bothering her in a way, she picked it up a fair bit, but she was still putting weight on it and didn't appear to be lame at the walk. Then I tried touching her again and pretty much anywhere behind her shoulder she would hump up again. She wasn't swishing her tail at all either. I walked her a bit she seemed okay, and then when I put her back she trotted and then cantered off.
I have no idea what this is. I have never seen it. I've seen horses arch away from pressure from the back but never this. I was thinking about calling a vet but from my experience while they are great at stuff like wounds, something like this they aren't much help with.
I know horses get sore backs but its never usually like this.