M.O.
I agree with McMama, your body may be telling itself to "hold onto" the extra weight while you're nursing. It's a design of nature so that in the event there was a famine, your body would have some fat stores to convert to breastmilk and feed you.
Focus on being healthy - eating well, hydrating well, exercising/destressing (all of which it sounds like you already are!).
Finally, the best way that I've found in breaking through plateaus is to mix up my exercise plan. They have a saying, "If you always do what you've always done, you'll always get what you've always got." Your body adjusts to your "routine" and seeks stability. So if you want to bust out of that plateau, try some interval training...it's not about being in the gym for 45 mins, it's about working HARD. I liked to do things where I could push myself hard for a few minutes then take a little break doing lighter work, then go back to the hard. Check out the Body for Life Book from the library. He talks a lot about how to keep seeking YOUR limits and pushing yourself. He's got a great program on how to work from 1(low) to 10(high), using intervals for both cardio and strength training. His philosophy is to get in and out of the gym so you can enjoy your life, not spend your time there exercising by putting in an hour on a treadmill.
Best wishes!