The first time my daughter did that I was petrified. It always happened when she was upset, and tired, and fortunately only a few times.
When I talked to my mum, she told me one of her sisters did the same,but then she slept and was fine thereafter.
That was what I was experiencing. I mentioned this to the baby nurse and she assured me it was fine.
But yours was far more frightening, and your story brilliantly told. I imagine you will always be on guard, but I hope your anxiety ends up taking ab it of a back seat.
Wishing you wellness all round Amy. 💜💛💜