Help with getting clothes to fit and manging my weight?

I'm a sixteen year old girl and recently my size 000 jeans won't fit. I'm 5'6 and weigh 80 lbs, I measured around my belly button and got 17 inches. I'm not anorexic because I know I'm skinny and have never tried to loose weight. Honestly I usually don't eat all day because I find it boring. I'm also a sports fanatic. I usually burn about 2000 calories. Recently I lost a TON of weight, and my 000 jeans won't fit. first question WHERE DO I GET JEANS TO FIT ME. You can see all my bones. My hip bones stick out about three inches, and you can count my ribs, This is when I'm not sucking in. You can see all my arm and leg bones. My second question is AM I UNHEALTHY. third is AM I UNDERWEIGHT an if so HOW DO I GAIN WEIGHT. One more thing. Last time I was at the doctor he said he would put me on a tube if I didn't stop loosing weight. I havn't seen him since.