Maybe it's fat converting to muscle? Also, often when you start a new program, your metabolism actually slows down because it goes into shock. That is, it tries to conserve energy. (I assume you know all this considering the field you work in ortho lol) Drink lots of water and stick to your guns. Sometimes if you do fall off the wagon, you can always do a day of straight protein, but I wouldn't worry too much about it unless you want to be dead serious. Just do an extra 15 mins on the treadmill instead. You'd probably be better off.