Just make sure you're getting appropriate nutrients to pass on to your child, don't worry about calories unless your body gives you signals that there's a problem. My doctor had me continue taking my prenatals while I breastfed. If you don't get the right nutrients for your baby, your body will rob itself to feed the child. Don't be overly worried unless you notice you're having trouble producing milk. I breastfed both my daughters on a 3 hour schedule and donated the excess (I had to pump for comfort; there was about 10 extra ounces a day). IMO, having a routine caused me to produce *more* milk, moo cow that I was, lol.
Motherhood: When changing from pj pants to yoga pants qualifies as 'getting dressed'.