My son is six months and I also work away from home 2 days a week. I don't think comparisons are usually helpful but I think it's pretty good if you have been consistently getting 6 oz with just one pumping session! From what I've read, when you drop a pump, your supply often goes down - sounds like that didn't happen for you. And your period can definitely affect supply - for some people, I think it's substantial.
And although that does seem to be not a lot of milk, if she is happy while you are gone, and her growth percentiles are basically average (was your doctor looking at the CDC charts or the WHO charts - the WHO (world health organization) are based on exclusively-breastfed babies, which tend to grow more in the first few months and then slow down.
You mentioned that she hasn't taken a bottle - does she get breastmilk while you are at work or does she do most of her nursing at night? If that's the case, and it's not new (meaning you've been back at work for a while), I'd worry even less because it means your daughter is regulating for herself.
So that said, here are my thoughts and my experience, if you do want to try to increase your supply (remember that your pumping supply is different from your nursing supply - the baby seems to be almost always more efficient). From what I've read (check out pumpmoms yahoo group if you want lots more ideas and advice), stress definitely plays a big factor -- no one really seems to know why, but for many moms, worrying about your milk seems to make it slower. I've definitely experienced that, and before/during pumping, I take deep breaths and try to relax and it seems to really help. I know it sounds totally new-agey, but it seems to have worked for me.
About 1 month ago, my pumping output also seemed to drop and it made me nervous b/c I'll be increasing my hours at work at some point. It was around this point with my first son that I also had a dip -- it never really came back, despite trying just about everything except for dom (more water, oatmeal, gatorade, hospital-pump).
I'm doing three things differently this time. When I'm at work, I'm trying to fit in a third pump session, which gets me 2-3 extra oz. The two bigger things are that I'm pumping every single night. I used to hate doing this but it's now part of my routine and I either watch TV or read blogs while I do it - which make it easier to leave the pump on for 30+ min (as long as it doesn't hurt). I also take shatavari about 2 hours before pumping at night and I've really noticed a difference - sometimes, it seems like I can actually feel my breasts filling up. That evening pump (extra time, shatavari, consistency - not sure which it REALLY is!) has allowed me to freeze extra, which has helped with my stress.
Hope those are some helpful tips - I'll end my ramble with remembering to trust your baby and your body. As someone else recommended, it could be worth following up with a lactation consultant if there is a good one in your area (do be sure to ask about her experience with evaluating supply and knowledge of pumping). It might also be good to follow-up with the doctor to find out if there is a concern about your baby's growth or if it was based on the amount of milk. That will help you evaluate the appropriate approach.