What Causes Excessive Weight Gain?

My 9 year old daughter has been gaining weight at a pretty consistant rate. She eats lean meats, vegs, and fruits with very little carbs except for complex ones. Her thyroid has been checked and is normal. She is extremely active on a daily basis. She has an odor, it is very strong at times. Very sweet and her feet smell horrible. She has had many yeast infections as well. We are very very clean and she showers daily. Do you have any suggestions for the weight gain? She weighs 107 lbs.