Hello dear, Weighing too little can contribute to a weakened immune system, feeling tired. You can check if you're underweight by using height weight calculator index which shows your body mass index (BMI). If your BMI is below 18.5, this suggests that your weight may be too low. There might be an underlying any medical cause such as an overactive thyroid. Have you been finding it difficult to make time to have a healthy, balanced diet with regular meals?Have you lost your appetite, perhaps because you're worried or stressed? Have you been trying to lose weight?