I would like to see a sample of your daily diet and workout routine in detail but first/best guess is your are eating to few calories causing you to lose muscle. When this happens while you may go down on the scale, you will actually get bigger because your body fat percentage goes up. Its good though that you are measuring because this is one of many things that lets up know where we may need to change things. You are 16 and naturally are going to have a faster metabolism and if your deficit is to much this could be one such effect of that. So if you want post those details and we can see what might be the problem.
However I would like to add if this is just a one time thing or isn't constant it could just be bloat or water weight, also since your a female time of the month issues could come in. So more detail again would be needed.