I breast fed all three of mine and I although I dropped a lot of weight at first, I couldn't really drop the rest until I stopped nursing. And, I'm glad I didn't. Your body composition and diet DO have an effect on the quality of your breast milk and if you aren't getting enough fat and protein in your diet, it comes from your body. Carbs aren't really an issue, but we're usually so darn tired, we crave them for energy to keep us going. Believe it or not, you may have lost a lot of muscle mass through the pregnancy and nursing, I did. I went on a strict hypertrophy/strength cycle for 4 months after my youngest turned a year and then went back to conditioning after that. It really helped my energy levels and I got a lot more after my workouts. I felt that I was just spinning my wheels trying to "burn off" the fat before then and my joints hurt and I was just exhausted.
Anyway, I hope that helps.