Is it true that if you breastfeed your baby, you can lose a lot of weight?

Depends. If you look at the specifics, creating breast milk for baby requires metabolic work on your part ( that is quite passive).Over 1yr your breasts will use your body energy to create roughly 500-1000 cal/day for baby. If you only consume the diet you need for your pre-pregnancy weight, you could drop 1-2 pounds a week due to breastfeeding alone.If you eat more you loose less.
Possibly. Weight loss while breast feeding is largely related to caloric intake and caloric usage (particularly exercise and activity)--as is true in other situations as well. Life circumstances (baby's demands, post partum healing, psychic stress , nutritional needs etc) may make weight loss while breast feeding difficult-but not impossible.