Flashbacks are defined in the fourth edition of the Diagnostic and Statistical Manual of Mental Disorders as a “recurrence of a memory, feeling, or perceptual experience from the past.” This phenomenon is usually seen in individuals with posttraumatic stress disorder (PTSD), but because of the overlap between borderline personality disorder (BPD) and PTSD, it is also often seen in people with BPD.
An individual experiencing a flashback will describe feeling as if they are not fully in the present moment, but instead are in a past (usually traumatic) event. They may describe actually seeing things that happened in that past event and experiencing the event as if it were happening now.
Many people are confused about flashbacks because of how they are portrayed in movies or on television. For example, in the movies you may see images of combat veterans "hitting the deck" and acting out combat traumas, completely unaware of where they are. While this kind of flashback (called a "full immersion flashback") can happen, usually people will still maintain some awareness of the present moment while they are in a flashback. So, even if you are experiencing a milder version, you may still be having a flashback.
Flashbacks are closely related to a class of symptoms called dissociative symptoms. Your therapist can help you determine whether you are experiencing other dissociative symptoms of BPD.
Allen JG. Coping With Trauma: Hope Through Understanding. American Psychiatric Association, 2004.
Brewin CR. "Memory processes in Post-traumatic Stress Disorder." International Review of Psychiatry. 13(3):159-163, 2001.