All of a sudden a pressure related pain in my left upper abdominal cavity sort of feeling the pressure up to my left trapezoid muscle. feel like this could be heart burn ?

I agree with: your analysis. The reason for the pain in neck and shoulder region is most likely irritation of the phrenic nerve.

Possible: It affects people differently . Since you have had heartburn before and think this is heartburn try a double dose of either Prilosec or Prevacid (lansoprazole) and see if it helps. It is also possible that you could have pulled a muscle if you are working out. It is in your abdomen to the left so no vital organs there.