My 4 year old son yesterday was complaining sayin his leg was hurting but I didn't really think much off it until today as he has been limping all day and crying saying he can't walk and when he sits down he needs help getting up. He says its his hip and it looks really stiff when he walks. What could it be? He has not hurt it its just come on all of a sudden he has also been feeling a little unwell the last couple days.