Not sure if this is what you mean @msg132, but I describe it as a feeling of doom and yes it does have a losing your stomach physical sensation along with it. It usually happens when I am almost falling asleep at night. It is so unsettling that I then have a lot of trouble relaxing and getting to sleep. I have never had it during the day or even associated with anything in particular, though.
I have been on Tamoxifen twice in my 7 years of anti hormone therapy (initially for 2 years and have now been back on it for 2 years after Letrozole for 3 years in between). It has only happened on Tamoxifen I think. Seems to have stopped at the moment so maybe your body does adjust and stop having that reaction after a while. I hope it settles for you too. It is a dreadful sensation. Checking with your doctor might be a good idea. Take care. xxx