You should call your vet - after 2 weeks of lameness, something's not quite right!! Hope it's nothing serious!!
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.
You should really call your vet.