It might be because he is sick. It also might be that he is injured or uncomfortable in some way. Try to exercise him with a quick lung (5 to 10 minutes) because if it is really because of sickness then you need to keep him moving at least a little bit (I'm not a real vet but I heard this FROM a vet). Check his body to make sure he isn't hurt or uncomfortable in any way. If this behavior goes on then you should really call a vet. I hope your horse gets better! :)
As Dunjasha said, it could be a lot of things, as the result of a "bad step", or a injury caused during working that apeared later. Anyway I recommend you to call a vet to figure out what your horse have :)
Happened to my horse too. there are a lot of things that could have caused the injury. He could've hit himself somehow or he could've hurts himself when standing up, (which happened to my horse) from a laying position and pinched a nerve. It depends if there's a swelling. Definately call your vet.