this happened to me with all of my pregnancies. In fact I was smaller just after I had them then when I first got pregnant. The baby takes the nutrients from you one way or another and if you arent giving it the nutrients it needs at the time it will take them anyway (from your backside!!!) lol
I always put the weight back on pretty fast though so this time I am going to be wary of what my body does and not "eat for my milk" once I have had bub as I am not one of those lucky ones that lose weight from breastfeeding.