My son is 21 months old. He's been walking/running for the past 8 months and has ben using his legs perfectly normal. Sinced yeatserday he hasn't been able to walk - he cries everytime he has to stand up or walk - only the right leg though. He did fall yesterday when climbing off a very low chair, but there's no swelling or bruising anywhere on his leg, ankle or foot. He does not cry when I rub or press ******n his foot, ankle or leg. What can be possibly be wrong??