I just got a reply from the pumpmoms@yahoo.com email list. Mary says, (with my emphasis)
The hormonal changes associated with your menstrual cycle will definitely affect supply - generally negatively at first. Your description sounds like the classic situation. In the week preceding AF, as hormone levels are building, your supply dips. After ovulation and the egg is not fertilized, AF begins and your supply comes back up. I could always count in lowered supply the week before I expected AF.

You can't do anything about the hormones, but at least you can be prepared, drink plenty of fluids and relax. You may get a little less for a couple days, but stressing about it will make it worse."

Thanks, Mary!

Here is the question I posted:
Does where we are in our monthly cycles affect milk production? If so, what's the pattern?

I just finished my morning pump session at work and got 4 oz when I usually get 2 oz, plus or minus 1/2 oz. I did pump about 30 minutes later than usual, and my son didn't nurse as long as usual this morning, but I would not have expected that big of a jump. After pumping, I went to the bathroom and noticed AF beginning. My son is 4 1/2 months old and I have had 1 light period and 1 close-to-normal period since his birth.

I did have some supply problems last week, but I attributed them to stress (his first week of daycare) and me not drinking enough water. If this is going to be a monthly pattern, I can adjust, but I'd like to know what others think.