Well for starters, 2.5 pounds of weight loss a week is quite a bit for the average person. That'll sometimes happen at first, but unless your very overweight, you most likely will not continue to lose 2.5 pounds per week for a long period of time. Also, you said you just started P90X which involves weight lifting. You could easily be putting on muscle which would cause the scale to go up or some people initially gain weight when they start a new exercise program which could also be the case. Don't stress about the scale. Focus more on measuring inches and taking before and after pictures. I've seen people gain weight but look better than before because of the muscle they've built and how toned they are. Weight lifting could also be causing you some initial water retention if you're not used to it, i'm not sure if you did weight lifting along with your cardio before. If you're gaining weight, but not toning up or losing inches, then you should check on your diet and make sure you're eating properly