Hundreds of reasons -- please don't worry about this. It could be time of month, salt content of food, your body adjusting and shifting around weight, amount of water retained, time of day, all kinds of things.

You might want to keep weight averages. Write down your weight every day, and then average it at the end of the week. (e.g., 200, 199, 198, 199, 197, 198, 197 will give you about 198). It's going to take time