Hi!
Try to focus on that holding your breath thing. I do the same thing and it actually makes it worse. It sounds like the anxiety of the flashback is definitely manifesting itself physiologically-- so just try to remain aware that this pain is anxiety, and don't let it cause you more anxiety.
Ground yourself.
You can do this-- it's going to pass soon. The more relaxed you can make your body, the sooner it will pass. THEN you can look back on it and try to deal with it, if you choose. Just focus on relaxing right now.
I hope this helps! I experience very uncomfortable body memories associated with flashbacks, dissociation, etc. It's a real pain, literally. You're not alone though.