What people think flashbacks are:
• Intrusive images/audio of your trauma that make you emotional and might cause you to forget your surroundings
• Intrusive images/audio of your trauma that MAY OR MAY NOT cause you to lose track of your surroundings
• Extreme unexplained emotions
• Fight-or-flight that won't shut off for hours, days, or even longer, wreaking havoc on your body (insomnia, chronic fatigue, dysregulated digestion, IBS, gastric ulcers, autoimmune or chronic pain disorders) because of soaring cortisol levels
• A startle reflex that goes off at the tiniest sound
• Powerful urges to self-medicate, self-harm, etc.
• Heart rate soaring, heart palpitations, chest pain, difficulty breathing, symptoms like a panic attack
• Muscle cramps and spasms from tension
• Pain. Like, extreme pain, especially if your trauma involved physical injury. Your body is repeating what happened in full detail, including whatever your nerve endings picked up, so you can feel like you're being shot, stabbed, tortured, raped, or anything else, over and over and over
• Other phantom sensations, feeling pressure or touch or blows as if they're really happening
• Non-epileptic seizures that feel like hell but can last as long as they damn well please without killing you
• Unexplained headaches, stomachaches, vomiting, tingling, numbness, sexual arousal, etc.
• Dissociation, feeling detached from your body, zoning out, a feeling of flying/floating/rollercoastering, feeling unreal, not recognizing your body, visual and spatial distortions, viewing yourself from above or behind or from a distance instead of vision originating in your eyeballs like normal
• Stiffness, paralysis. Not being able to move, losing the ability to speak, maybe even the ability to breathe
• Dizziness, loss of depth perception, fainting
• Sudden, irresistible drowsiness
• Becoming childlike or confused, temporarily losing track of your identity or memory
• Sensory hallucinations, anything from visual to taste
• Nightmares, not just of the event but of endless bizarre and awful things that make you feel the same way the event did
There's more, and I'm sure every person has a unique combination of symptoms, but that's a start.
Use this stuff in your fanfiction. Post-traumatic stress needs realistic representation!