Stress isn't a bad or a negative thing, nor is it a "weakness." It is vital to our everyday welfare. Stress is a trigger for such things as adrenelin secretions which are necessary for everyday functions and sometimes for survival. It is the awarness and management of stress that is often important, so that it be can recognised and controlled. The stressors themselves can then often be managed and distributed to positive effect.
If you want to raise a smile with the interviewers, you can say that the last time you felt stress was outside sitting in the waiting room! However that stress is useful in enabling you to focus on the topics for discussion. You are much more relaxed now you can talk face to face.
It will show self awareness and a positive understanding of stress rather than suggesting it is any kind of weakness.