She's very likely in a growth spurt, and that extra suckling does stimulate you to make more. Depending on how old she is, your supply might be switching to being more demand-driven, rather than hormonally driven. Eat a lot of oatmeal, and add flaxseed or Brewer's yeast (not regular yeast--go to Whole Foods or an herb shop and get Brewer's yeast) to it. Sprinkle the yeast on everything you eat. Make cookies and throw in a couple of heaping tablespoons of the yeast. That'll help.
You're right about not wanting to supplement. If you give her formula, then your body doesn't know to make up for that feeding unless you're pumping, and then what was the point of the formula? Babies also tend to like the ease, so she might begin to snub the breast and you'd become more dependent on the supplement. Supplementation CAN hurt (or destroy) your nursing relationship.
Go to kellymom.com and read about this. Kellymom.com is an invaluable resource for breastfeeding moms. You're doing a GREAT JOB, and keep following your instincts! Since this is your second, you know that it does get easier, especially once you get your supply really established. If you feel like she's working and not getting much, you can also do a few breast compressions (directions on the site) to get her started.
I used a sling with both mine, actually. I had a Hotsling, and I wore my son EVERYWHERE. It's the easiest thing in the world since you can just latch her on and go about your business. And yes, there were days that I barely did anything but feed the baby. :) Mostly, though, I'd pop him into the sling and my daughter and I would take off. He'd nurse or sleep most of the time, and when he wanted to look around he could from the sling.
You can do this! Good for you choosing what's best for your daughter.