Thanks for posting your query.
Your doctor may be right, your chest pain
may be due to muscle sprain
& strain as you exercise hard which causes stress
on muscle, you may confirm it by pressing painful area if it feels tender then it is musckuloskeletal.
You can take acetoaminophen/naproxen/diclofenic to relieve pain.
If you want to be sure then visit your doctor and get chest x-ray to exclude any lung disease.
Hope it helps.