Just that! Pump more! :) You sound like you're doing the right thing if you want to pick up your supply. If you're barely filling the bottom of the bottle, you're probably getting an ounce! That's pretty good for 11 days PP and especially since you're feeding at the breast as well.
It's hard to judge how much you're making by what you pump when the baby is at the breast too.
What kind of pump are you using? Have you tried hand expression?
i agree with mature enough. At 11 days, i didn't have much extra yet. you're on your way! Also, the baby should hit a growth spurt soon. All of those cluster feeds will naturally boost your supply, and once he settles back down you'll probably be able to get more.