Disrespectful attitude

I have been a customer since 2014, and I have never been treated the way I was treated an hour ago by a young lady at YDE Eastgate , I was just returning a skirt that was small, she gave me two receipts and a pen to sign, I signed and returned the signed receipt and asked if the other one was mine to keep, she kept quiet as she was busy listening to the other customer being attended by the other cashier and I asked again if the receipt was mine to keep, she answered yes and I left the teller and she spoke out loud to the other teller about me and I went back and apologised if I had done anything wrong to her, at that moment she just rolled her eyes, I have never felt so bad in my life, I am 41 years and she is about 19 years but the attitude she gave me broke me. She could have just waited for me to leave the shop but she deliberately did loud enough so that I can hear it all, and I heard it.