K.
9 months to put it on, 9 months to take it off. Yes, seriously, it can take that long. I put on 45 lbs with my first pregnancy and was within about 5 lbs of my pre-pregnancy weight at around 7 months postpartum, but I was still a size up from "normal". Lots of time even if your *weight* is the same, your body shape has changed.
My suggestion: cut yourself some slack and go buy a few key pieces of clothing that actually fit you and that you LIKE. You will feel a lot better about yourself when you like the way you look. You may have to look for some different styles that flatter your new body. Chances are you'll have another baby at some point and you'll be glad that you have some larger clothes that flatter you for your first year postpartum.
Also, don't forget that if you're nursing, it's probably your breasts that are keeping your shirts from fitting! One more tip - there's nothing like Pilates for getting your core strength and muscle tone back. I ended up getting a DVD that I could do at home after putting my son to bed (it was too hard to get out of the house).