Oh no!! So sorry to hear this :-(
I agree that it may be back pain as my friends pony went from being just like yours to unridable! They got the physio out and within a couple of weeks she was back to normal and they haven't looked back.
Good luck and I really hope she's ok x
We're now on our 3rd lorry from Central England horseboxes! Luke has always been very easy to deal with, honest and really knows his horseboxes.
A few of my friends have also bought and sold through him and have had nothing but good things to say.
I personally would highly recommend them :-)