Pain with breathing is not normally heart related. You may have a lung infection, bronchitis or pleurisy. All will have chest pain with breathing. If it hurts worse when you breath in deeply, it's most likely not cardiac in nature. You should see a doctor if you're concerned.
I agree with Jon and would like to add that it could be a muscular problem in the chest.
It's easy to pull a chest muscle, even coughing will do it. These can be painful when breathing deeply or shallow and can last for months.
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