In Jungian psychology, a complex is a cluster of emotionally charged associations, memories and expectations that can organise your response before you have fully considered the present situation.
The term doesn’t mean that you are a complicated problem somebody needs to solve.
One comment. An unexpectedly large reaction.
Someone questions a small decision. Suddenly you feel young, defensive or desperate to prove that you know what you’re doing.
The present comment matters. So may the earlier experiences it seems to gather around itself. A Jungian account would ask about that charged cluster rather than assume the visible event explains the whole response.
The point isn’t to dismiss you as irrational. It is to make more of what happened available for understanding.
The Hidden Hero’s Journey uses symbolic figures and questions to explore patterns in its own way. Association starts with a single connection. Inner parts offers another language for responses that can pull in different directions.
The International Association for Analytical Psychology’s The Theory of Complexes explains the concept within analytical psychology. It is a useful next read when an everyday label such as "overreacting" leaves too much unexplored.