My friend had pain under her left breast that she thought was heart related. It had nothing to do with her heart, it was stress induced.

Stress, pulled muscle, gas (wind) all will cause pain here - or you may even have knocked yourself there without realizing & have a bruised muscle or bone - if you're pregnant then worrying like this won't help you or the baby - & during pregnancy there'll be alot of aches pains & odd feelings so relac - that would be best all round.

I get pains under my left breast when i'm having an asthma attack. This could be related to u not breathing well. Maybe a cold, or something. U may want to go see a doc.