I have been eating healthy for years now and that has helped me feel better, but I keep having trouble keeping the weight down anyway, despite a very active lifestyle. I have been doing one of the Yoga workouts every morning for about 2 weeks now and then maybe take a walk later or do a Move Video and I've lost almost 5 lbs already.
I am not a Mom, just wanna say that right off the bat, so I have no first hand experience, but from everything I have heard a hardcore workout can make your milk supply go down. I also was told that if you make sure to workout throughout your pregnancy and then just keep going it won't affect you, so I deducted from that, that it must have something to do with the drastic change. Perhaps if you slowly, carefully work your way into it, your milk won't go down.
I read that you get more after burn (which keeps burning calories when you're not working out) when you do workouts that are a min. of 30 min. long. The longer the more after burn. So that might be something to consider also.