Yes then, most of the time if your horse suddenly looks like it broke a leg it's an abscess. They are incredibly lame and it's super scary the first time you see it. Usually they burst out somewhere and the horse is just as suddenly almost totally better. Possible the rock caused the abscess, or possible the rock just got lodged in the hole from when it burst. Your farrier can probably tell you. Also, you might want to call the farrier instead of the vet for this one. Not all of them handle stuff like that but a lot of them do.