It's called an "involuntary memory".
And they're very normal. Dr Jennifer Wild, a clinical psychologist at the Oxford Centre for Anxiety Disorders and Trauma, told BuzzFeed that almost everyone gets them.
"For a while, after a trauma or an embarrassing event, most people have these memories," she says. "Some will get a lot." It could be a month or so of regular flashbacks. Some people get more, some people get less, a few get none at all.
What's happening is to do with the way memories are formed.
"Your brain retains the information about the traumatic event, and the feelings associated with it," says Prof Chris Brewin of the Institute of Cognitive Neuroscience at UCL. It's a sort of punishment/reward system, he says, and it's doing an important job: "It reminds people of important experiences – of danger or humiliation", to stop you from doing the same thing again.
You learn by associating experiences with outcomes. This is classic psychology, going back to the great Russian pioneer Ivan Pavlov, who used to ring a bell before feeding his dogs. Eventually the dogs began salivating in expectation of food just at the sound of the bell.
The same thing happens in traumatic or socially embarrassing memories. Your brain brings back the unpleasant sensations – the fear or the shame – when it finds itself in a situation similar to the original event. And with traumatic or embarrassing memories, says Dr Wild, the effect is pronounced. "In these situations, we get pumped full of adrenaline, and that heightens our awareness. That makes memories more vivid, and then when something later reminds you of that situation, it can bring the memories back."
They recur most often in the days immediately after the event, but can be revived years or decades later.
They can be triggered by an external stimulus – for instance, says Dr Wild, it could be that a particular shade of cheddar yellow in the Waitrose cheese aisle reminded me of something that was present on that fateful day in 1998. Or could be internal – some way you're feeling, or simply your train of thought.
For some people, it can be a real problem.
Most of the time, for most people, these memories aren't an issue, beyond being unpleasant and embarrassing. But there's a spectrum. Involuntary memories, which most of us get, can become intrusive memories, which are symptoms of PTSD, depression, social phobia, and anxiety disorder.
The two are on a spectrum. "It depends how distressing the memory is," says Dr Wild.
Prof Brewin says that for most of us, our involuntary memories don't tend to recur, but "in the disorders, the same memories come over and over again; usually not more than three or four different ones, sometimes in a cycle. One memory can trigger the next, which can trigger the next." You get stuck in a loop.
Dr Wild says that "if it's distressing, if it stops you from going out or makes you avoid social interaction" then your intrusive memories could be a symptom of a mental health issue. "People can be crippled by them," she says. In severe cases, traumatic memories can be so vivid that they make sufferers believe they are back in the situation which triggered them.
If you're experiencing lots of intrusive memories, and it's distressing, there are things you can do.
"The best way to deal with them is refocusing attention," says Dr Wild. "It's what we call stimulus discrimination. Give your environment a lot of attention." It means acknowledging the similarities between the situation you're in now and the original memory, but also focusing on the differences. A traumatic memory of a car crash, for instance, could be triggered by bright lights like the ones you saw a moment before impact. But by concentrating on the differences – the bright lights aren't coming through a car window – you can weaken the link between stimulus and response.
"So if you're in the cheese aisle at Waitrose, and you have a flashback to a some embarrassing thing you did," says Dr Wild, "remind yourself that you're in Waitrose, that you're not in the embarrassing situation." Eventually, hopefully, the stimulus which triggers the memory will become more and more specific.
People who suffer from PTSD and the other disorders "often get very good at avoiding their triggers, the words or images and TV series and so on", says Prof Brewin. Trigger warnings on violent imagery is "certainly sensible, because if there's no warning, it can trigger quite severe episodes".
However, as with any health issue, if you're really worried about your intrusive memories, you should seek help.
Prof Brewin points to Improving Access to Psychological Therapies (IAPT), an NHS initiative designed to get people who need mental health support the care they need more quickly. Your family GP is another good first port of call for mental health issues.