Day 9 - hmmmm

This was the second morning that I woke up with my lower back being stiff.  In all fairness, I couldn't get it loosened up yesterday at all.  When I got out of bed, I was honestly thinking about taking the day off of Insanity and trying to loosen up my back.  With my plan and focus, though, I knew I couldn't take the day off, so again I was hoping that the stretching portion would help loosen me up.  Today was "Pure Cardio" day.  Starting off with the slow job, I started to regret getting started.  However, after getting through the first round of warm-ups the ache started to subside, so I pushed myself a bit harder.  The stretching did help and got me through the rest of the workout.  I still feel a little tight as I write this, but I feel better than what I did yesterday.  I really sweated my ass off this morning and pushed myself hard and really tried to give extra effort.

Now, here's my puzzlement.  I know my doctor is really focused on me losing weight (hence the pretty damn strict diet).  It was also mentioned about me losing fat %, which is something that I had already known that I needed to work on.  My wife and I were talking about this last night.  At some point, there's going to be that plateau of where I'm not losing weight.  There's also that point of which I'm building muscle.  So with both the weight and the fat percentage being areas of focus (and him mentioning that I really cant come off the diet til I get down to 175 - on his scale, which is about 170 on mine at home.... keep in mind, I weigh in late afternoon and fully clothed at his office), what happens if my weight isn't/doesn't drop but I burn off most of the fat?  Anyone that is really into nutrition, etc, please help me out here and explain.