Can Extreme Shivering In A Child While Swimming Be Due To Low Body Weight?

My little one is small for her age (used to have a feeding tube and eats on her own now, but not enough when she is sick. She has been sick a lot this spring). Anyway, she loves to swim but gets so cold. She has to get out after 5 or 10 min. b/c she is shivering so badly. I let her stay in 15 min today b/c teh water was warm, but she scared me b/c of how badly she shivered. Is shivering this badly b/c of her low weight or could there be something else wrong? She is 2.5 years old, normal height, 24 lbs.