Heather's boyfriend Matt served combat duty in Iraq. After returning to the US last month and reuniting with Heather, Matt shared the details of the battles he was involved with and the trauma he experienced while serving. Heather has begun to experience intense periods of anxiety and feels as if she is "in a daze". It is possible Heather has developed as a result of listening to Matt's experiences.

an Acute Stress Disorder

Explanation:

An Acute Stress Disorder -

It refers to the initial stage , where the person starts to get some sign of stress and anxiety , is referred to as an acute stress disorder .

The stress can be triggered very easily by some action , talks , incident or any past expedience  .

The disorder can lead to various signs like stress , tension , anxiety , panic mode etc.

Hence , from the given scenario of the question ,

The correct answer is -

an acute stress disorder  .