Does anyone have any idea what might be going on or any suggestions til his appointment (we have a vet appointment Thursday). He is 4 1/2 years old. Not overweight. Extremely strong. Very active. He suddenly just started limping on back left leg one evening. He isn't whining in pain however he is irritable. Three days have passed he doesn't seem any better. I checked his paw pad and nails and they seemed good. I don't see any swelling. He is only using all 4 legs when he absolutely has to. He is slightly off balance when going up steps. He hasn't been as active. Just kind of laying around.