According to hans selye, resistance to stress is lowest at the "alarm" stage of the general adaptation syndrome.
The alarm stage is otherwise called the fight or flight reaction. At this alarm stage, your brain sends a crisis flag to different parts of your body, which tells those parts that you're in a risky circumstance. As your body activates itself to respond, you can pick between a fight or flight response. Resistance levels to stress are lowered at this stage and after that these levels start increasing.