Is armpit pain a heart attack?

Unlikely: Usually heart attack pain is felt in the left jaw, down the left arm, and/or across the left chest and may be accompanied by nausea, shortness of breath, dizziness, palpitations or passing out. Sometimes it can be a vague sensation in the chest or like heartburn, and sometimes there's no pain at all. Armpit pain would more likely be muscular or a swollen lymph node. If in doubt, check it out!