FWIW, we had the same thing happen to us with our Citibank card. One day, it was declined for no reason. When I called them up to ask why, they told me that there was a hold on my card because of some sort of fraud (someone tried to use it illegally???), and a new one was on the way.
They would never tell me who used it or where the fraud occured/came from so that we could take some precautions. Sounds like it’s S.O.P. not to divulge this info, but I don’t like it, either.
