First, congratulations on your miracle and for pumping at work! It's hard work!!!!
The progesterone only pills work fine for a lot of women, but really any hormones have the potential to affect your supply. There are lots of things you can do to help increase your supply. You touched on a bunch of stuff so forgive me for just running down the list as I caught them please:
Feeling Full - around 3-4 months out, your supply is switching from hormone driven to supply and demand driven. It's very common to not get engorged at this point. It doesn't mean that there's no milk or nto enough milk, just that your body is regulated to your baby and your baby's schedule (or the pump schedule).
How many bottles are you sending to day care? How full are the bottles? Is your care provider using alternative methods to sooth the baby when fussy or just putting a bottle in?
The average nursing mother can pump .5 to 2 oz from BOTH breasts combined per pumping session, so it's common to need multiple sessions to make a bottle. (www.kellymom.com).
To increase supply, take a nursing vacation on your next day off - spend the day skin to skin in bed or on the couch. Just cuddle and nurse, nurse, nurse. Drink lots of water. Eat all the oatmeal you can handle - cookies count too. (there's a great recipe for lactation cookies floating around the internet).
If you decide you need to go that route, there are herbs and teas that can help increase supply, things like fenugreek, mother's milk tea, and I believe blessed thistle.
How often are you pumping? When DD first started sleeping well for me and was going down around 8 pm, she'd sleep until about 3 am. I added a pumping session around 10-11 PM and it quickly grew to be a productive session for me.
I can't say for sure whether the bleeding you're seeing is related to pill withdrawl or the return of your cycles. If it is an actual period, it is very commong for a lot of women to see a supply drop between ovulation and the start your period.
And of course, it's always possible that it's a growth spurt and the baby is trying to ramp up your supply to meet new demand levels.
Best wishes, you're doing a great job!