Re: One defunct breast
This link covers lopsidedness and what to do about it: http://kellymom.com/bf/got-milk/lopsided/ Basically you want to increase stimulation to the underproducing breast until production on that side increases. This can be difficult to do when baby hs a strong preference for the high production side. Try different positions, try offering the smaller side first, try offering it at the end of feedings, and if the baby won't do the job on that side, pump every time he refuses.
Can you post a complete weight history for your baby? Maybe we can reassure you. It's completely normal for a baby's growth rate to vary, with the most rapid rate of weight gain in early infancy, and declining throughout the first year. (That's why the growth curves are CURVES, not straight lines!) Many babies will drop percentiles in the 4-6 month window, and particularly active babies may begin to drop percentiles earlier.
Coolest thing my big girl said recently: "How can you tell the world is moving when you are standing on it?"
Coolest thing my little girl sang recently: "I love dat one-two pupples!"