"You were there, not I," is very proper English. However most people, at least in America, are very careless and don't use the nominative correctly. So most people would say "me" in that type of sentence. Here's another example: "She didn't do it; it was he," is perfect English, but you won't hear anybody say that in America. They'll say, "...it was him." There is even a joke about this: "Knock, knock." "Who's there?" "It is I." "Oh, the English teacher." And that joke goes back at least 50 years, so this is an old problem.