You probably have a very fast metabolism. I second the decision to have a food log, write it all down. Increase carbs and proteins from healthy foods and lift weights. A three day full body routine should be perfect for beginners. This should put on muscle mass. with proper diet and exercise you can probably put on 20 pounds in a year our two. Remember its a marathon not a sprint. And don't worry about being "buff." You're a female, and unless you take steroids it is physically impossible to be buff like a male, you can only have a female body. Remember there is nothing more aesthetically pleasing that a fit female with muscle tone and an athletic body. Muscle is real curves, and curves are muscle. Fat women claim they have curves but in reality they just have rolls which is disgusting. Good luck, you can definitely do it and you'll look great too.

To gain weight you need to combine starchy carbohydrates such as bread, cereals, pasta, rice, corn, potatoes or sweet potatoes with proteins such as meat, poultry, eggs, cheese, nuts or sea food.

Peanut butter sandwiches and cheese sandwiches are both good weight gaining combinations.

Snacks in between meals often help as well. Supper calories are very fattening as with most of the day behind you they are not burnt up.

The protein in the above combinations can also help to gain extra muscles.

keep a food diary for a week and write down everything you eat and the time. you might find that in fact you are NOT actually eating enough.

Go see a doctor.

Could yourself as very lucky! Enjoy food as long as you can.