paid in full for the watch but was told to come back monday

I bought a watch by sterns, it only needed to be resized, paid for it and told the lady that I will be back after few hours to fetch it. When I went back that very same watch I bought was sold to someone else and I was told that I should come back monday even though I paid for it. like what the hell is that? I would have understood if I didnt pay for it in full. I am really ****ed