You've gotten good advice here, I'll just throw in that I too could/can eat and eat and not gain, and in terms of your overall health and *the nutritional quality of your breastmilk*, you don't want to lose the weight too quickly. Rapid weight loss will stress your system further, and for me, there was a definite correlation between losing the rest of my baby weight and (I think) the fat content of my milk suffered. There was a marked shift in my son's growth curve at that time, and I don't think it was coincidental since he was still exclusively breastfed, nor do I think it was a growth spurt since we'd made it through several at that point without a growth curve shift. Rereading my original response, (I'm editing now) I don't think it was a supply issue, it was a quality issue... The idea that breastmilk is breastmillk no matter what you eat is just not true. You have to be just as mindful as when you were pregnant I think.
For example, a study published in the British Journal of Nutrion stating that eating organic milk and dairy improves the nutritional quality of breastmilk: http://www.organicconsumers.org/articles/article_6245.cfm
Breastfeeding (without supplementation) burns 500 calories a day, FYI.
Worry about the quality and variety of food you're eating and ignore the scale. If you eat good quality foods and don't eat junk the only weight you put on will be exactly what you and your baby need, for pregnancy and nursing, to stay healthy.
Good luck and congratulations!