My 15 month old son got suddenly weakness on his legs why? He started yesterday,it happens twice. He was on my couch and he got down and try to walk but the legs got weakened and he fells. Today I sit him on his high chair, and when he finish to eat I put him down and he got his legs weakened again, I have to help him to stand up so he can start to walk again, he walks but it takes him a little bit to walk firm and strong again!! Why? Just to add, he had fever 2 days ago and receive his shots the 21 of last month and he fells and bump his head on the floor 4 days ago!!