Hi J.,
Congrats on your new baby! It sounds like you are doing great! I would worry about having different pump output on each side. That happens to many women. My sister said with her babies that she would pump 1 ounce on one side and 5 on the other. It is pretty normal. Her babies also preferred the slow side when they were infants (because the fast side gagged them) and as they grew they started to prefer the fast side because they were too impatient for the slow side. Continue to feed your baby equally on each side and don't worry about how much you are getting on each side. If you continue to have the lump, you do probably want to have that checked by your OB. It doesn't sound like a plugged duct because it isn't painful and it has been there for a while, but any persistent lump should be checked by your Dr.
I am jealous about how much you can pump! 15-20 ounces per session is AMAZING!! I never pumped more than 2-3 ounces per session. Everyone is different - some women pump only a little (like me) and others pump a lot (like you). How much you pump isn't generally an indication of how much your baby is getting as the baby is much more efficient at getting the milk out than your pump.
Here is a link to a pumping tips article that I wrote which you may find useful. Many of the tips are for increasing supply - which it doesn't sound like you need, but you may find some of the other information useful - http://www.pumpingtips.com.
J.
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