It may be enlargement of the muscle from exercise. Get a stress test. Take aspirin for now.

slow down on exercise... make sure you reduce any stress in your life... could be stress related... try doing some stretches, 5 minutes, 4 times a day... just some simple stretches, like reach your toes,,, or w.e be creative... if you are worried about the heart then you would be getting a CT scan.. not a MRI.. and try relaxing and taking your mind off of it, and then see if its reduced at all... if so you might just be worrieing too much. try those, if it doesnt work within the next 3-4 days you could go see a oc about a CT scan