Re: insanity, and loosing milk supply
I would say at least increase your calories by 1,000 which includes the 500 calories for BF'ng.
I haven't done P90X but have done couple rounds of Insanity and I incorporate it twice a week with four days of weight training. You'll be starving honestly if you do Insanity or similar activities at least five times a week.
Insanity alone burns up to 700 or so calories in an hour; and that's not counting whatever calories that's used to restore your body. Water intake would be critical as at least for me drinking 32 oz of water is usually done during the workout not to mention you will be covered in sweat (if you sweat easily especially) the first 10 minutes of the warm up. An ounce of water for every two pounds of your body weight might be a good measure.
Food wise increase your lean protein intake, good fats and healthy source of carbohydrates. If you find it hard to add more protein into the diet I would consider using protein shake or meal replacements. Especially with Insanity you'll want to eat no later than 45 minutes after the workout to replenish your body and basically have the body switch to a restoring mode.
Insanity is great for both speed, strength and leaning out. But also it does require you to do a lot of jumping and moves that require explosive movement so if there are any concerns I would speak to a professional before starting this type of exercise. Good luck
Dad to two precious girls