Well, since you asked, I think I would have to vote for over-reacting.
Obviously, I was not there to experience the whole situation, and I might have felt the same in your shoes.
But, I just don't know if it would be worth my time to challenge or argue that any further.

For a movie theater, I would probably carry a small flashlight in one of my pockets, so I could find the door in the event of an emergency... and then get to my bag/supplies in my car if needed.
Or course, if you're talking about a "big city public transportation" scenario where you have no backup available... that would present a whole different challenge.

One thing to keep in mind is that dealing with the "public" can be a really tough job sometimes; and there is no telling what that manager may have had to deal with recently (or earlier that day) that may have triggered a negative reaction to your bag.
Unfortunately, she may not have handled the situation in the best way; but, unless she was just way out of line, I would probably just try to give her a break and move on.