Any chance you know a good equine vet who could do an MRI for you? Might be something there that the X-rays aren't picking up?

Good luck, hope you find some answers soon!

It could also be pano, there are many things it could be including a strain. By asking everyone, you're going to get many answers.

If it continues, go see an orthopedic vet instead of lay people without names, degrees and who are throwing their opinions out there including me.

I hope it's growing pains too.