Make sure you do everything in writing - the law is based on receiving written notice within x number of days. Calling the bank has no legal standing.
Great suggestions, folks. I will act on them accordingly. And great instruction and insight.
I had a thought that it might not be as odious as someone actually "hacking" my system...they might have just gotten ahold of the number from a previous purchase somewhere, and used it. In other words, simple direct bold theft, nothing as daring as hacking me.