It dawned on Kara that something was wrong, when she tried to clear her anger, as she was accustomed to, and accidentally looked in the mirror. The area near her eyes was covered with crimson veins - barely noticeable - an ordinary person would hardly have pointed it out right away.

Kara had a predisposition to the Thinkers Guild for a reason, and as a result, Supergirl immediately made a hypothesis.

The history of this multiverse changed when Kara decided to figure things out on her own.