An EKG has a low sensitivity rating for dx of heart issues not related to rhythm disorder. If there is chest pain when coughing does not indicate a heart disorder;.
More likely it is a respiratory issue. Blood clots in the lungs (pulmonary embolus). The pain is very sharp and stabbing in nature, has a sudden onset, and is worse when taking a deep breath (referred to as pleuritic chest pain).
Another possibility is having a clot in the pulmonary vessel. You should see a doctor is elminate those serious condition.
Or it could be costchondritis. That is inflammation of the cartilage related to the skeletal rib cage. Usually as a result of an injury.
Anytime you think the pain you're having could be caused by your heart, it's not over-reacting to have it checked out.
Once a heart attack is ruled out, then the doctor can look into what might be causing the pain.
A lot of times, pain like that can be caused by something as simple as sleeping on your left side for too many hours at night. Or it can be caused by sleeping with your mouth open, and your lungs drying out.
In any event, you need to get to the doctor, and have another EKG, and then go from there.
I hope the best for you.