What Causes Slight Pain Under The Breast?

often I get slight pain under my left breast... I have been worried about this for years, no answers from test, [all good!] I'm now 47 [been experiencing it since 16'ish?] always active as a young person, not so much as an older woman... but it always "gets me" and scars me... normal for some people? stress seems to trigger it, physical or emotional?