Mystery Lameness

SO today when I got my horse out of the pasture he was limping on his right front leg. The pasture he is in is REALLY muddy, plus yesterday we were cantering and he stepped in large pit of sand with his right leg. There is not any heat in his leg, the more he excersises the less he limps, but if stands for a while then I make him walk he limps really bad for a few steps then after a while the limping decreases. Does anyone no what this is or how I can fx it?