Brief Answer:
Better be concerned as you need to see a Doctor and get an EKG

Detailed Answer:
Hi.
Thanks for query, albeit short and specific.

Noted your history and understood your concerns.

Pain in crease between left armpit and left breast can be due to the following:
Muscular: will increase with movement of left arm.

Most important is to get an EKG done to rule out any pain of cardiac origin.

Can even be an inflammation or infection of the underlying tissues. Then there will be locally raised redness and tenderness.

Please correlate the symptoms with the points mentioned above.

However it's always advisable to please get an assistance of your Doctor for clinical evaluation, examination and further investigations as needed to get a proper diagnosis and treatment.
I hope this answers your query. Any further queries are most welcome.

Regards

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