I've lost 18 pounds in 4 months w/o trying. i moved to japan but still eat junk food sometimes. i ride my bike to work. normal or am i sick?

Could be normal: Losing 18 pounds in 4 months with increased activity and possibly dietary change may be normal, especially if you were sedentary before. I don't know where you moved from or how you were eating previously, but typical japanese diets differ greatly from those in the us. They generally eat more vegetables, lean proteins, etc, than we do. Personally seeing a physician may provide answers for you.

May be normal: You don't mention what your weight is. If you are living a healthier lifestyle and were overweight then your body may be adjusting to a healthier weight. The most important thing is how do you feel? Good energy, sleeping well, good mood? Then you're probably fine. One east thing to check is your body mass index (find a calculator on line). As long as your bmi is around 25-29 that's good.