I don't think you should worry about trying to lose weight while you're breastfeeding - but I also think this is a question best asked of a professional.
Here's a link to a breastfeeding site: www.breastfeeding.com.
This site is by moms and for moms; you can get help and support with breastfeeding issues as well as other parenting and mom/child questions. (It's a great resource - my daughter is able to successfully breastfeed because of the support we found here and through La Leche League.)
You should be able to get this question answered there, either by one of the lactation consultants or in the FAQ section or on the community forum.