go to web site--diagnose-me. look for your condition. it will have the answers. at your age the chances of heart problems is almost non existent. plus trying to get a doctor to do a full workup with the insurance paying for is more non existent. have your tried any nsaid pain relievers like ibuprophen? nsaid stands for non steroidal anti-inflamatory drug. but that web site should give you help. costochronditis is common. when a word ends in itis, it means inflammation, like tonsilitis.
You need more testing ,of course go back and ask for more tests. EKG can't show everything. Tell about your pain, they must give the total testing for you. It might be ultrasound etc.It is your life. Take charge.My best for you.