Not sure if this is the same as the "sinking feeling" I am reading about in other questions,
I will have an "oh no" moment for no apparent reason, followed by what feels like a sinking, but it is more between my chest & stomach than up in the chest. Followed by a mild feeling of dizziness & moderate nausea. My breathing becomes a little shallow. I can continue to function. There is no particular set of circumstances that seems to bring this on.

I am prone to slightly low blood pressure, pretty good heart rate. This feeling can come while up right, seated, or lying down. Passes within 1-2 minutes.