One day while I was riding in the subway and I bent down to reach my shoe, I suddenly felt this crushing pain right under my heart. I instantly thought I was having a full-on heart attack, but the pain lasted for less than a minute and then it was gone.
What had actually happened was that a section of my rectus abdominis muscle on the left side went into spasm.
Why this particular muscle would spasm I have no idea, but in recent years I have also experienced periodic nocturnal spasms of both my calf muscles (gastrocnemius), too. So it seems it may be due to some intrinsic muscle disease that for whatever reason only shows up in adulthood.
Anyway, as with all weird symptoms, this one needs a name and I have decided to call this one an abdominal heart attack. Since that first one, it has continued to occur several times a year.