You could keep your calories up, but eliminate processed foods like sugars and starches.
When you're nursing, it's important that you still eat fat because otherwise your body will burn your fat in the milk. That sounds like a good idea, but your fat is where toxins are stored, and your baby then gets more junk than he should.
If you eat meat, vegetables, nuts, plain yoghurt, fruit, and lots of water, your fat intake and calories are still high enough, but those junky calories like sugar and refined starches are missing. You could go down to the 1200-cal level if you were eating the right foods, and lose weight that way.
This was my experience and what I've learned in taking LC courses.
Kerri