http://www.fertilityfriend.com/Faqs/Fertility-charting-while-breastfeeding.html
Can I chart while breastfeeding?
Before you get your first period, you may experience a long period without ovulation, especially if you are breastfeeding several times a day and during the night. The associated long cycles and erratic temperatures can be frustrating and confusing. If you start charting when you begin to see signs of increased fertility, however, you may be able to identify your first ovulation. Pay particular attention to your cervical fluid as it will usually attain a more fertile quality before your first ovulation. You may, however, experience several patches of eggwhite cervical fluid before your first postpartum ovulation.
If you have already gotten your first postpartum period, you should be able to chart while breastfeeding and see your fertility cycle without much difficulty, though you may have a few added challenges. Some breastfeeding women, for example, notice unusual cervical fluid and temperature patterns. The combination of breastfeeding hormones and night-waking (or early morning waking) can cause erratic temperatures and either multiple patches of fertile cervical fluid, or a decrease in cervical fluid. While you may get some rocky temperatures, you should still be able to get a reliable chart interpretation and a recognizable biphasic (ovulation) pattern if you chart as accurately as you possibly can.