I’d be careful about taking medical advice over the Internet, especially with regard to this problem.
I’m glad you saw your doctor. Chest wall pain is fairly common and it is important to exclude serious causes like heart attack. It can be due to spasm of the muscles in the chest wall. But a common cause is costochrondritis. Your ribs join on yo your breastbone (sternum) in the centre of your chest. But there is a joint between each rib and the breastbone. These joints can easily become inflamed, like in arthritis, and cause temporary muscle aches. There is not much to be done for this apart from waiting for time to heal wounds and taking medicines to relieve pain and swelling. Tylenol, aspirin and ibuprofen can all be helpful, as can applying ice in 10 minute sessions. Some gain benefit from creams like Rub A535.
If this is your problem, it will heal with time and you can be reassured that your muscles aren’t going to start ripping all over the place. People with costochorndritis tend to get better with time. But you should go back and see your doctor if you notice symptoms like difficulty breathing, nausea, vomiting, feeling light-headed while you are having the pain; especially if it comes on while doing strenuous activities.