(I just thought of this too, and it is the case in my case) You may be gaining muscle mass. Instead of losing weight
, you are staying the same by developing muscle mass. (I actually began to gain weight because of my training routine. I was training like a body builder which is really all I knew. Now I concentrate on cardio, cardio, cardio! The treadmill, the stationary bike, the eliptical machine and the stair stepper are my best friends in the gym.