Am I Too Underweight?

So I'm a 16 year old female. I'm not worried or anything but in 2 pounds underweight. My bmi is like 18.2 or some. I also have kyphosis which causes weight loss, but so does anxiety. Whenever I walk on the store for a long time my back hurts & my heart beats really fast & i get scared it's going to stop eventhough it's probably not because i have no heart problems. Does weight loss cause these problems?

You mentioned weightloss..so whta was your previous weight?..or have you lost substantial weight in the last couple weeks or so?

130- 140ish

Lost 20lbs. in 1 month. It was very quick & sudden.