One of those automated announcements at the boarding gate says something like:

"The economy cabin on Delta 1234 to Tallahassee does not offer meal service. Delta customers are always welcome to bring food on-board."

To me, a consumable (food) should be allowed but I have only ever tried it as a third item in a bag that was clearly from the airport. Perhaps if the OP had a plastic shopping bag it wouldn't have caused an issue.