I Am An 18 Year Old Female. I Am 5

I am an 18 year old female. I am 5 feet 4 inches tall and weigh 93 pounds. That's about a 16 bmi. I have always been extremely skinny and underweight. I eat 3 balanced meals a day, sometimes more. The only exercise I get is playing tennis 2 times a week for about an hour, so hardly any. I don't know why I am so underweight, but here is some more information. About me that might be helpful. I have had constipation for as long as I can remember. I can't remember the last time I was happy. I am always extremely tired. Why am I so thin? Is it possible for someone naturally underweight, meaning that there is no reason for then being underweight and that they are completely healthy? My doctor had never been of any help about this.