This just happened for me yesterday, and I remembered this thread, but couldn't come up with a plan at the time. So I decided to do it today to try to keep last night's awfulness from happening again, or at least have more options to try first.
First things first

Pay enormous attention to how everything I'm doing feels and affects my emotions, thoughts, and intensity of flashbacks; adjust as needed and stop anything that’s making it worse to move onto something else

Take anxiety meds

Journal with my muse/imaginary friend

Shut myself in back 'safe' room

Optional: robe, heater, heating pad, electric blanket, fluffy pillow, or open windows if fresh air feels good
Daytime options

If it’s sunny and warm, hoopwalk around favorite nearby greenway looping a lake is an option

If it’s open, local free art museum and sculpture garden or one of the other cheap or free art museums

Punching bag
Other options

my calm and soothing ‘It's easier to think about Dolphins than to NOT think about rhinoceroses’ playlist; or some custom white noise selections I made of favorite ambient sounds

Reading Diana Wynne Jones, Terry Pratchett, or Tad Williams’s ‘Otherland’ series

reading some other e-book I have waiting to be read, paying attention to what increases stress and steering clear

Reading what I call my ‘Sennsucht’ wish-fulfillment stories or writing new ones

Ukulele

Playing ‘
The Beautiful Dream’ or ‘
Lunar Flowers’ on my iPhone, these gorgeous free games evoke a peaceful feeling I like to think is similar to zen gardeners raking their gravel into patterns, or some other hypnotic repetitive soothing act

Scream or cry into pillow

cutting out shapes from cardstock using templates to make 3D shapes for collages

cleaning something really well

make gummy candy

downloading, sorting, editing pictures from camera
Journal questions

What do I feel?

How can I make it easier?

What do I wish?

How can I give myself my wishes in fantasy?

What has worked before, even in the distant past?
Love,
Holly