Well, the first thought that comes to mind is to call and see if you can switch dress sizes, since the dress hasn't come in yet. The company might charge a little more, but it would likely be worth it. You could also see if another lady participating in the wedding has ordered a size that is too big & instead of having it altered, trade dresses. Believe it or not that is what happenend to me this Dec. for my sister's wedding! Another brides maid had ordered her dress too small, (A size 12 instead of a 14) and I was, what 6+ months preggo?, and had over ordered in size...a 14 to be on the safe side so we just traded dresses & it all worked out.
Also, you should be loosing quite a bit of weight just by breast feeding. (It's great when you don't have to do much and the baby just sucks it off you!) I'd call and ask your doctor if you can even excercise yet AND if doing so would hurt your milk supply, BUT if he/she says it is ok, I recommend doing an activity...like walking on a treadmill or an eliptical machine...where you sweat for 10-15 minutes two times a week...no more than 3. AND STAY HYDRATED!!! Being in the sustained sweat for 10-15 min twice a week really helps kick up the metabolism and burns calories...but I would DEFINITELY ask your DOCTOR first.
Good Luck & Worst case cenaro, find a creative seamstris who can enlarge the dress for you! Whatever happens, weddings are lucky & it will all work out! Best Wishes!
P.S. It is dangerous to loose more than a pound a week.