Now, first of all, I'm doing the P90, not P90x (I'm quite overweight and unfit, and I know I'd be better off starting at a reasonable level), but I intend to work my way up to it.

2 Weeks ago I finally decided to stop being lazy, and I got up an hour earlier each day (6/7 days a week) to exercise.... before that I never exercised. So I'm doing the P90 program, and eating quite well. Even using this great online program called fitday.com to track what I eat and do. It suggests I burn about 3200 cals a day, and I'm eating about 1600-2000.

After week 1, I lost 6 lbs! But After 2 weeks, I went back up 5 pounds, and I didn't really change my nutrition or workout much.....what could have happened?

Perhaps some more details?

I get up at 6:30, jump straight into my workout, then have breakfast about 7:30...either a cup of nonfat cottage cheese (68% protein), or some cereal like special k (high fiber).

At work, I usually eat a banana/apple/some veggies around 9am. lunch is around 11:30...quite varied. deli turkey sandwich or leftovers from dinner.

Around 2:30, maybe some veggies or nuts (cashews or pistachios).

Dinner around 5:30, then I avoid eating for 3 hours before bed, usually 9:30-10pm

It frankly sounds like you're not eating enough.. your body needs food in order to keep your metabolism up and burn fat. If you don't get enough calories, then it shuts down and you start packing it back on.

The other possibility is that through not eating/drinking enough you lost a lot of water weight, and then it came back through stabilization if you haven't changed your diet.

The key to weight/fat loss is remembering that you have to take your time doing it.. there's no way you can lose 100lbs of weight over a couple of months, considering it took 20, 30, 40? years to put it on. Take your time, and make sure you're getting the calories you need. Eating 2300-2400 calories a day is better for you than eating half the calories your body needs to exist.

If you eat well, you keep your energy up and will keep going.

The nice thing about being a big guy is that you don't have to eat like a rabbit if you're doing all the exercising as well. You can still have a decent amount of food, and burn the calories as you go.

The calorie level is a little low but I'd only increase it to 2000.

One thing to remember, muscle weighs a lot more than fat. Muscle is more dense, so it can fit in smaller space. I usually don't like people to track their weight, it can be upsetting. Instead I like to tell them to track their body fat. There are tons of ways to do it and a lot of scales at a reasonable price to do those functions.

There is always the tape measure way. Just google it, there are tons on Body Fat Calculators.