Duck limping on one leg! Help!

[LordDuckWings]

Just about twenty minutes ago, my male call duck was chasing me around the yard, biting at my ankles behind me. I had never stepped on anything, but I think he might've hit his leg on a cinder block in my backyard. About a meter away I stopped to see him lifting one leg. I got worried, and brought him to my deck to see what was wrong. Nothing seemed to be broken, and he looked fine when I was checking him. When I was looking at him, he jumped up quickly and flew to his mate. I ran towards him to see if he was injured, but he just started limping again. He is only limping, nothing else. Picture included.