The BMI sounds just like my son last year. His BMI was so low and PS sent a letter home (beginning of K, so he was just 5). I was annoyed at the letter in particular, but evidently it was a state thing. Anyway, we brought it up to the ped and they weren't at all concerned because this was his normal. There was not change drastically in his habits, so the weight was "normal" for him. He was around 35lbs then. Our 3 year old neighbor girl weighs the same as him even now! :) Now, he didn't have the leg pains, etc. and I would want to check that out. But i wouldn't be worried about just the BMI.