Think of the most awful event in your whole life. Then imagine reliving that time and time again while feeling powerless to stop. Your heart rate quickens, thoughts race, and breathing begins to race as your body begins the fight or flight response.
This is an example of how someone with PTSD might feel when triggered. Every person responds differently, but there are some common factors for all who suffer from this disorder. Although logically in a safe place, this person feels the panic and vulnerability from a past experience. Physical sensations accompany mental terror which makes this type of anxiety difficult to face alone.