A Flashback is a involuntary vivid, often distressing experience where you relive some aspects of a traumatic event or feel as if it is happening right now.
According to PTSDUK, flashbacks can take many forms and can vary in intensity and duration. They can be experienced as vivid and disturbing memories that feel like they are happening again in the present moment. These flashbacks can be accompanied by intense emotional reactions, such as fear, anxiety, or anger. They can also cause physical reactions, such as a racing heart, difficulty breathing, or a feeling of detachment from the body.
Mind have created a page on PTSD which discusses the symptoms and causes of flashbacks as well as giving you tips to coping with Flashbacks
RAINN have put together tips to coping with Flashbacks in the moment and also how to prevent Flashbacks
ChildLine’s page on PTSD and Flashbacks can help young people understand what flashbacks are and why they are experiencing them, how to seek support and advice on staying safe, including how to make your own safety plan