When my son runs/walks he often looks as though it is hurting him - as if his shoes are 3 sizes too small (they are not). He claims not to be in any pain, but it LOOKS painful - so much so that other parents comment and worry while watching him play soccer/basketball etc.
He is a pretty big little guy - at around 100 pounds and quite tall - which, considering he was a six pound baby 8 years ago, is a lot of growing pretty fast. He is not at all fat - still a bit of a baby belly shape, but otherwise a very athletic little guy. Just a very painful and odd looking limp/gate when he walks or runs.