@MrsWinslow - Breastfeeding slows down the regrowth of your bones after pregnancy, but does not prevent it. Yes, your body will use calcium to make milk for baby. In addition, estrogen levels are low during breastfeeding, and that also keeps calcium off your bones.
But according to the NIH (National Institutes of Health - government agency), within about six months after weaning your baby, you will have regained any lost bone density.
Breastfeeding advocates actually claim that breastfeeding *protects against osteoporosis in later life. I'm not sure how good their evidence is, but they say that when you regain bone density after breastfeeding, it often comes back even denser than before. So don't let that worry you - breastfeeding is good for mom (in most circumstances) as well as baby!