He looked kind of puzzled and asked "So how come women don't keep a "go bag" with spare pants and underwear? That's what guys would do."
I replied "I really have no idea. Maybe because they would worry that the spare wouldn't match their outfit?"
"Well, blood doesn't match anything!"
But it does make me wonder why we don't do it that way.
Some of us do.
Back in my early 20s when I would regularly have the Week Of Doom And Despair, I kept spare undies at work and in a ziplock at the bottom of my shoulder bag, and I did not own a single pair of white pants. Everything that I was willing to wear out of the house was black or denim, and wouldn't show damp watermarks if I had to rinse it in a hand basin. 
And I thought I was the only one!
DH asked me what women do when "accidents" happen and I explained that we all become quite good at wetting, blotting and drying our pants - and that it is possible to get them dry enough that you can sit on them until they dry completely and no one is the wiser. 
{For me, it's not "The week of Doom and Despair" but rather my "Stuck Pig Imitation".}
"The War Is Begun" or "The Battle of No Grandkids"
My father and I have a whole code because he's so grossed out. No white out, cuz its the gross out board.
Tampon: Ammunition
Pad: Body Armor
When I Start: Deployment
Blood Clots: Under Fire
Blood Clots Big Enough To Give The Family Name: Casualties. (Or "BFF's Namesake")
"Why were you in the bathroom so long?"
"Double Casualty."
My best friend and I speak each other's language. She calls it "The Moon Is Full" and her dad isn't a pansy so she can call her supplies by their names.
Also she once told me there was no way my clot was as big as my thumb. So I saved one for her in a jar. Her words:
"Oh... wow. I think its bigger than you told me.... is this my namesake?" (explanation for why above phrase is used.)
If that isn't gross enough yet, this is the BFF that I have changed "supplies" for. She has mobility issues and broke her dominant hand. Her non-dominant hand has issues so I removed her tampon and put pad in her undies for six days. She told me that this is proof I'm her sister, a non-sister would never do that for her.
Then she looked me in the eye and said "One of our moms has some explaining to do..."