Not all women lose weight while they're nursing. The theory that you'll lose weight while breastfeeding is predicated upon the calories in vs calories out equation. See, you need to burn 3500 calories to lose a pound, which translates out to 500 calories a day. Coincidentally, that's exactly the number of "extra" calories you need while breastfeeding. So, in theory, you will lose a pound a week by breastfeeding alone.
However, there's some stuff missing from this equation. Namely, that you cannot defeat nature. Our bodies are to begin with programmed to retain some fat stores so that we can nurse. When you're actually nursing, that tendency seems to kick into high gear for some reason, and cause them to hold on to weight. It doesn't apply to all women, all the time, but there are enough women who've told me they can't lose weight while nursing that I suspect it's fairly common.
The advice you've already been given is great, of course. If this is your first, I can tell you it will probably get better when baby starts in on solids, which might be as soon as tomorrow or as late as four months from now--the range of normal for that is 4 to 8 months, & go by baby's cues not the pedi's suggestion on that.

I
think that 4 months is also prime time for another growth spurt. If she's doing not much more than sleeping or eating, she's probably fixing to grow.
As far as Atkins, some people have said babes don't like the taste of ketones if they start spilling into your milk. Since I'm not convinced of the safety of that, or of how good my nutrition would be in that case, I try hard not to go into ketosis.
If you're worried about milk supply, you can also pick up mother's milk tea or try supplementing with brewer's yeast (the latter I have found at Wal-Mart). Other than that, just be sure to keep hydrated & get enough protein.