Rene'
Since the Dr. has ruled out nearly everything, is it possible that she is just petite? I met a little girl years ago when she was 2. Tiny, 13 lbs. I babysat with her for 5 years and she was fine, no health problems, just a tiny, petite little girl. She's in her 30's now and has 3 children of her own. Hopefully this will be the outcome for your little one too. God Bless.