If ECG is normal,It could be a condition called Costochondritis , inflammation in your sternal muscle.
Check your posture in work,driving.some research data says this can be seen in smokers and people who consume less water ,who have inadequate sleep.
Next Steps
gentle chest muscle stretches, ice pack application.posture correction,adequate sleep and water,sun exposure.
Health Tips
consult a Physio for better posture and exercise advise