Why does my heart feel so thump-thump?

This morning on the subway, I suddenly couldn't see in front of me, felt dizzy, had chest pain, and had difficulty breathing.

I fainted and urgently went to a large hospital's neurology department for blood tests and an electrocardiogram, and they said everything was normal.

I usually had orthostatic hypotension, but when I was examined at the hospital, my blood pressure was normal, and I was told there was nothing specific to treat.

Currently, my symptoms have improved, but occasionally I experience chest pain and a feeling of tightness in my chest.

Why are these symptoms occurring if there's nothing wrong? ㅜㅜ