Frustrated

There could be many reasons. Some things to consider? Did you reduce your calorie goal too drastically? Are you making healthy food choices even if you are staying under your calorie goal? Are you staying adequately hydrated? Have you had a physical recently to ensure there are no physical issues like thyroid, diabetes, etc? Is your scale correctly calibrated? Did you weigh at the same time of day both times? Could you be pre-menstrual? What kind of effort is exerted when you do the P90x?

Finally, a week isn't necessarily that long, depending on your lifestyle before you started. Don't let it get you down, frustrated, or off track. I personally have had weeks where I worked so incredibly hard I just knew the scale would move LOTS, and it wouldn't move. So I would back off a little, add a few more calories and the next week seemed to make up for it. Hopefully that is the case for you!

I think of it like this.

You are replacing fat with muscle. That means your total weight is not changing, but you've exchange a pound of fat for a pound of muscle. That means you are shrinking! There will be less of a fat pad around your internal organs. Your clothes will be looser (eventually). Even the muscle you have will get leaner.

There may even come a point where you are much smaller, much leaner, but actually weigh more than some other time in your life when you were larger, but weighed less. That has happened to me. I weigh 10-15 lbs more than a few years ago, but actually am 2-3 sizes smaller. Kind of a mind trip.

Think of a fatty piece of steak. Can you picture all the marbling (the fat veins)? Now think of a lean cut of meat without all the marbling. You are the steak- still the same weight, but entirely different- much leaner and much healthier.

My advice: stop weighing yourself! Start measuring. Keep moving.