So this whole episode came on after a XC schooling day and he's never had the problem before? That is really interesting. While it would be perfectly normal for a horse that does not have much turnout to have this problem frequently, yours obviously did well until now. Did the vet say anything about why this suddenly became a factor?
Glad to see he's doing better. Even if you only hand walk him for 15 minutes twice a day, the movement will help. Don't beat yourself up about not getting the vet out sooner. I find the learning curve with horse care to be much steeper than the riding curve. We just do the best we can for our equine buddies.