When doing heavy cardio, I get a sharp stabbing pain in the middle my left pec. I believe this is pericarditis I have had an EKG+xray-nothing detected

Not Pericarditis. Sharp pain during exertion is unlikely to be cardiac or pericardial in etiology. This is most likely musculoskeletal. Pericarditis discomfort would not be present only during "heavy cardio", it would be present persistently both standing and especially lying down. Try taking a Motrin or Advil or Alleve or Tylenol (acetaminophen) 30 minutes before working out and see if that helps.