This event occurred at the Circle K gas station on Geneva Rd in Wheaton, Illinois (USA) at about 12:30 pm on Friday, May 28, 2021. The staff in this business have always been very nice to me; open and friendly. Today I was shocked by the behavior of the sales person. She was sitting when I arrived at the register; I couldn't see her initially. She left me standing there for a time, then huffed loudly, got up, and yelled at me, ‘YEAH?'. I asked if she was talking to me, and she yelled very rudely, "I don't see anyone else here". I pointed out that the staff in that business had always been friendly and helpful, and that I wasn't used to being spoken to so rudely. I finished my business, and before I was about to leave she was yelling at the next customer. He had a problem of some sort; he was speaking quietly and respectfully and she was yelling at him. Physically, she appeared to be early middle age (I could be wrong); medium height, and rather thin; African American, and had on the red T-shirt uniform of the Circle K. There were no other employees that I noticed; at least behind the counter. I'd like to keep going to this store; it's convenient, and as I said, everyone has always been really nice. Frankly though, she was pretty scary; unstable, if you understand what I mean. If I see her there again, I doubt I'll go back.