Disappointed with poor customer service

Greetings.

I am writing this text with such disappointment. Earlier today I visited Food *****'s Market at Medlife Mall in Port Elizabeth. I had boughten FLM flavored water thinking they were the plain sparkling water at FLM Figtree also in Port Elizabeth so this afternoon I had the chance to take back the water for an exchange and FLM Medlife Mall was the closest option. When I got to the store I was met by an African lady who didn't mention her name. Firstly I couldn't find my slip but because I thought it's the store's brand it wouldn't be a problem exchanging the water but when I got there the lady demanded my slip or she couldn't be of any help to me, frustrated as I was I searched my wallet for the slip and I eventually found it. Secondly when I found the slip I was told I didn't buy the bottle of water at their FLM store and that I should return it where I bought it which I still don't understand. I would like to know if this behavior is acceptable because I have never experienced such inconvenience?