The only diseases that are associated with eating too little are those that are associated with getting few nutrients for your body. You may feel that you are eating too little, but if your body is getting enough calories, vitamins and minerals for it to perform its basic needs, then you will be fine. You may eat less than other do, but that may be a good thing. This is because the majority of people eat more than they should and subsequently gain weight. You also may have a slower basal metabolism than other people. This means that you may need less food than other people. The first question I would ask you is how much you weigh. If your weight is normal for your height and sex, and you have not gained or lost much weight recently, then you are probably eating the right amount of food.
This is the type of problem that should be handled by your primary care physician
. I suggest that you schedule an appointment today. Bring with you a diary of the foods that you have eaten and any weight changes you have recently had. This way you can be sure that your diet is providing your body with enough nutrients.