Loosing too much weight?

I am a sopre on a track team and I am a "long sprinter" (800 runner) and I basically do the quick and hard workouts of a sprinter but they are lengthy like distance. I have been in track for 5 weeks and right before I got this dress for a special dinner. It was supposed to be a very tight fiting dress and it was! (about size 5) and when I tried it On yesterday, it was falling off of me everywhere! So I was curious and tried on my skinny jeans... They were loose! Now all my things are too big for me. I use to be 5'7, just over 150 and I'm done growing. Now I am just at 115!! Is this an unhealthy weight loss??