Sounds like a back injury , maybe an impinged nerve, maybe a break. You need a Vet consult.
There are also some swine diseases that could cause this. If it is possible to quarantine the animal, until it can be checked out by a vet, it couldn't hurt. Just use caution. Many swine diseases can be transmitted to people. Erysipelothrix rhusiopathiae is one of them. It is bacterial, can cause lameness, and is transmittable. That does not mean it is what you hog has, but you should use precautions until you know for sure.