I am also breastfeeding. My daughter is 7 months old and I've lost a total of 57lbs since her birth... I have 100 to lose though. She has severe food allergies (dairy and soy), so my diet is very restricted however.
My doctor said a weight loss of 1-2 lbs a week is the maximum I should lose or my milk supply will be compromised. Don't restrict your calories too far because your body needs 300-500 calories to produce quality breast milk.
Your hormones are completely out of wack for the first 6 months post-partum and for several weeks after weaning is complete so don't beat yourself up about slow progress.
Making sure your body is getting the proper nutrients is a must... calcium, iron and vitamin D levels are quick to drop while breastfeeding. So make sure you continue to take your prenatals.
You should also schedule an appointment with your doctor to have your thyroid levels checked as lots of women have thyroid imbalances post partum which can lower your metabolism.