I am about to start p90x, but this nutrition plan included seems like a lot of extra work and stress and I am wondering if I need to mess with it. I am a 5'7" female weighing 125 pounds. I am already in pretty good shape (cardiovascularly and physically) from regular exercise and near the lower limit of my normal BMI range so I don't have "pounds" to lose. I want to do p90x mainly to tone and build some lean muscle.

I understand the nutrition plan would be vital for people with a lot of fat to lose or who don't know much about nutrition... but I feel like I eat well already. Only problem is I do eat a lot of carbs but obviously it hasn't been an issue in making me gain weight. Do I need to mess with this part?

You'll get results from the exercise section alone. I would not totally discard the nutritional section. If you eat a lot of carbs, it's possible you are not getting the ideal amount of protein, and you could use the plan to incorporate more protein in your regular diet.