When you have PTSD, your nervous system is constantly on high alert. It gets danger signals from the brain, but it can't interpret the reason for those signals (medical trauma as opposed to interpersonal trauma). It becomes so overstimulated that it sends physiological danger signs back to the brain, and the brain doesn't always interpret them in ways that make sense logically, since the nervous system doesn't even know what it's afraid of. So the brain basically interprets it as "EVERYTHING is dangerous!"