Fat, Skinny, Fat

I'm assuming you're female. This changing feeling is pretty normal. If other people say you're thin, than that's a good indicator. But, for a more accurate answer, compare your body against others of similar age and body type who are thin, but *healthy*, and who are eating properly. If you have a flat stomach and your upper legs look decent, you're probably doing perfectly fine.

You can weigh yourself on a regular basis, but don't worry about day-to-day fluctuations. Look at what is happening over the long term (weeks/months). If you ever need to go on diet, look into *low-carbohydrate* nutrition. Stay away from any low-calorie or low-fat diets. Never starve yourself.