Hi E., I am an experienced nurser (with my oldest I was in the military and still nursed for 2 1/2 years), so I have a few suggestions as to "cause" and "solution". Cause is the simple part. You know the maxim, the more they take, the more you make. I'm sure supplementing with formula is at least partly to blame for the drop in your supply, unless you're still pumping just as much as before to keep the stimulation going. Here's another possible cause. I noticed both times around that at about 8-9 months (or when baby was taking more solids) my sensitivity to stimulation and therefore my supply took a brief dip. I think this is fairly common (especially with the return of "mother nature" every month). Another possible cause - it may very well be that you're not pumping very much any more, but I'm positive that when your daughter nurses, she's getting plenty - The stimulation from a breast pump and a baby are very different, no matter how much R&D the pump company does.
Here are some tips and some advice. Nurse, nurse, nurse. The more you nurse, the more you will be able to make. Actually that ties into my other advice too, hydrate, and schedule. Make sure that you are drinking plenty of water all the time, and try to work your schedule so that your daughter is receiving mostly solids and water during the time you are at work. Or rather, arrange your daughter's schedule. I know how difficult it can be to try to arrange a work schedule, so work with the one you have the most control over. When my oldest was at about 8 or 9 months, he was only taking one bottle a day of pumped milk and the rest of the time was getting solids and water. They're old enough at that age for this to work. I nursed him in the morning before I left and the moment I got home. Then he nursed several more times before bed with one more solid meal in between. This way he got all of the milk he needed and all of the solids. In this way I managed to keep up because I didn't have to pump as much and my body "learned" the schedule and so would produce the most between the hour I got home from work and the hour I had to leave in the morning. (This could also be what your body is already doing, in which case, I would try a different pump, actually I recommend manual pumps - the Avent Isis series - because you can manipulate the suction on a very fine level and most closely simulate your child's nursing rhythm.)
Hang in there, your child won't need even that one bottle during your working hours soon. You only have a few months to go until she's one year old, then you can nurse only when you get home. I'd encourage you to nurse for as long as you can and don't draw an arbitrary line at one year. Yes, the AAP recommends at least one year, but did you know that the World Health Organization recommends at least two? Think about it.
Best Wishes
D.