How much does your hemoglobin count drop?

I ask this question because I gave today. Periodically the Red Cross refuses me because my red cell count is below their limit, around 117. Periodically they accept me because it's above their limit, which is 125. Today was one of the acceptance days. But it got me thinking.

It's not hard to work out that the days when I fail the test are after a period. But I'm darned sure I've never lost as much blood from natural causes as the Red Cross takes from me. It's not like I mistrust them or think the new low would be dangerous. I'm just curious. If a mere menstrual cycle can drop you 10 points in the count, what does giving about three cups of blood do?