I lost weight with both my babies, but I gained it back when I was stuck sitting around nursing and feeling hungry after the births. As long as you aren't trying to lose weight, and you are eating well, I wouldn't worry about it.
That's what happened with my daughter, I weighed less right after I delivered her than when I first got pregnant, but I gained like 30 pounds while breastfeeding and continuing to eat like I was still throwing up half of it... Gonna try to avoid that this time around.