HELP!!!!!!! :( 16 year old girl and have lost 20 pounds unintentionally!! Was already underweight!! :(?

Hey guys! I am 16 years old (5' 4) , and just over a month ago I weighed 120 pounds. I was at a pretty healthy weight, and all of the sudden, two weeks later i weighed, and I was 107, and it has just kept dropping and dropping ever since! Now I am down to 99.5! :( I am really scared. Like really. :( Somebody told me that losing weight is a sign of cancer and I am terrified. My mama had cancer 3 years ago, and it was the worst thing I have ever seen anyone go through. It was so scary. :( Anyways, yeah, I have been eating EXACTLY the same as I ever have! Not trying to lose any weight, I have been eating until I am full. I actually went to the doctors, and she evaluated me. She did the tremor test for Hyperthyroidism and everything, and she was almost POSITIVE it was hyperthyroidism. They did a blood test, and it came back negative. So, I don't know what to think! Now, she has made me keep a food journal for two weeks (I will be done in a day) and I have to go back for another check-up! I haven't had very many symptoms. I am just kinda shaky all the time, cold, and constipated (sorry guys! Tmi!). This is really kind of scary for me, so anything you guys can do to help, would be great! Thank you SO much!!!!