How Dare You Make Me Do Math?!

I’m really bad with confrontation.

Cashier: “Your total is $26.15.”

Me: “Okay.”

I hand her $41.15.

Cashier: “What are you giving me all this money for?!”

Me: “I— I— I— Change.”

The cashier gave me a dirty look. She called her supervisor, took a long time to count out $15, and shoved it into my hands. The whole time, she held my items behind the counter. I have no idea how I offended her so badly.