Ladies I AM LIVID!! what would you do

So today my SO and I met my parents (and baby - they were babysitting) at Costco to renew our membership and shop around a bit. We picked up a couple of things like baby wipes, baby clothes, socks and he got some underwear. While we were checking out my card decided not to work for some reason so my mother paid for my 100$ bill. I told her I felt bad and insisted I go to the bank to give her the money and asked where there’s a bank around the area. My SO had 100$ bill on him and gave it to her.

WHEN WE GOT INTO THE CAR HE ASKED ME IF I WAS GOING TO GIVE HIM BACK THE MONEY.

AM I WRONG FOR BEING UPSET? I assumed because we live together, have a household and raise a child that our finances wether he buys or I buy is a mutual household thing.