Underweight question?

I am a 23 year old female, weight 105 lbs and am 5'7". I have been skinny my entire life and it is hereditary b/c my mom and sisters are all skinny. My problem is i have always been between 115-120 lbs. And here recently I don't know why my weight has dropped. People at work have noticed and said something and so has my family. I have not dieted at all. I have been through some health scares this past month, i had to see a pulmonologist (lung doc) but turned out everything is ok, my CT scan of my chest and chest x-ray is fine. This whole past month i have felt like i couldn't breathe and have been to ER alot b/c of it and that's why i saw the pulmonologist but they said everything is fine and it is probably anxiety. I went to the doctor last week and weighed 107 lbs. I went back to the same doctor yesterday on the same scale and it said i weighed 105 lbs. Is this happening to anyone else? I have been trying to eat extra everyday just to prove to my mom that i am not intentionally trying to loose weight, like i have a eating disorder or something, and she says she believes me, shes just worried. I am too. Why I am now all of a sudden within the past month dropped over 10lbs?