Some people are just naturally thin, and that's not a bad thing actually. I've known women who were your height and taller and weighed what you weigh -- people come in all varieties. As for the weight loss in the last year, don't know why that is -- skipping meals? Getting more activity? 18 is a very active age. Since you say the doctors have evaluated you and found nothing wrong, and you believe they're right, the only way to gain more weight than you body naturally settles in to is to eat more -- add a meal, for example -- and if you do resistance training you will add the weight as muscle, not fat.
You need a calorie surplus to add weight!
You need about 250 - 500 extra calories per day to help development of healthy muscle.
Try for the 500 calorie per-day to add about 1 pound per week. Try to get those extra calories come from high-protein foods, not from white bread, sugar, soda and fast food.
Try to consume about 0.55 gram of protein per pound of your body weight per day.
Include vegetables, lean proteins, dairy, whole grains, fruits and healthy unsaturated fats. Choose starchy vegetables, like sweet potatoes, potatoes and peas, and serve them with leafy greens. Higher-calorie fruits include bananas, mangoes, papaya, avocado and pineapple. Roast lower-calorie vegetables, like broccoli, cauliflower and eggplant, with coconut or olive oil to boost the calorie count. Whole grains, such as brown rice, quinoa and oatmeal, add healthy fiber and nutrients that support muscle growth.
As for protein add skinless poultry, lean beef, eggs, white fish, salmon, shellfish, tofu or dried beans. A good 5- to 6 ounce gives you enough protein and extra calories. Add protein for snacks from string cheeses, cottage cheese, deli turkey or hard-boiled eggs. A protein-rich snack that also has some carbohydrates about 30 minutes before and after your workout, which can boost muscle growth and repair. Whey protein mixed in milk with a banana is convenient, but whole foods like tuna salad on whole-grain bread or a roasted chicken breast with a sweet potato are also good.
If you use whey protein I suggest only buy whey concentrate not isolate!!!
Hope this helps.
Probably you're working out more than your body consumes. This condition is not absolutely abnormal. You can try gaining weight by drinking protein mass gain shakes.