Low carb is fine when nursing, as long as you don't go too low.
Like any dieting while nursing, the best way to approach it is to figure out where you are NOW--weigh yourself, and write down what you eat now. Then look at how many carbs you are currently eating in your average day. Then start to slowly lower your carbs(or calories if you are a calorie counter) each week by a reasonable amount. Like calories, try dropping 100 a week, re-evaluate after a week; in carbs maybe drop 20 carbs this week and see how you feel, and how your weight is effected, AND how it effects your milk. Instead of using Atkins and dropping your calories SUPER low, and then slowly build back up to a reasonable level where you lose your weight and feel your best, work backwards, and just slowly lower your carbs until you find the level that works best for you, your milk supply, your PCOS and your weight....
Hope that helps!