Weight loss without increased exercise or decreased caloric intake?!

Weight loss without increased exercise or decreased caloric intake?

I'm a 17 year old, 5'7" caucasian girl who can't stop losing weight. I've gone from 145 lb to 135 lb during the past month, and I'm not dieting, and nor have I increased my exercise amount. Though, I have done some significant changes to my lifestyle-- I'm no longer at school (I just graduated), no longer doing track (~20 hours a week), and I don't dance anymore (ballet, jazz, and tap)...
Could my unexplained weightloss be attributed to the changing of muscle to fat? I've noticed my jeans don't fit as well any more, and my bras are starting to seem bigger than they used to.
I've changed my diet only slightly...I'm on an exchange-program in Germany for 4 weeks, but it's not like I'm eating totally differently.

If anyone can figure it out...lemme know, I'm at a loss for words! :D

Please go to a doctor right away. There are two possible causes for this: Diabetes and Hyperthyroidism.

With diabetes, you will also exhibit symptoms such as an unquenchable thirst and sleeping a lot.

With hyperthyroidism, you will have constantly cold hands and feet, sometimes anxiety and trouble sleeping.

Both conditions can turn serious if not caught early enough.