The “oh shit too much” feeling

Does anyone know “scientifically” or can explain what causes that roller coaster stomach drop feeling and then getting terrible anxiety and fear during a trip? I NEVER had that happen to me for the first couple years then it hit all of a sudden during an acid trip in 2020, and my trips have never really been the same since.

My original acid trips were full of fun and laughter, but when I had that first anxiety trip it was just plain awful and I almost blacked out. I’m NOT trying to put down or talk negative about psychedelics, just more curious and want advice from any experienced psychonauts. Thank you for being understanding 🙏🏼.