Ask your PCP to ultrasound your gallbladder. Is the pain related in anyway to your diet? Fried or fatty food make this worse? Pain associated with cholecystitis usually manifests in the midline of the upper abdomen, but can radiate like heart attack pain. Your work up seems to have ruled out pericarditis (my first thought) and pleuritis (you didn't describe pain on respiration so I didn't think this was a likely Dx). I am betting on the gall bladder. Interesting an ECG abnormality (S-T interval elongation) is associated with gall bladder disease. No explanation.
Disclaimer: I am not an MD, but I did stay at a Holiday Inn Express
Seriously, have the gall bladder ultrasound.