Pain Under Arm, Shoulder Pain, Nerve Pain. Cardiac Issues?
Thanks for the query.
Your pain don`t appear to be due to cardiac problem, as cardiac originated pain appear on left side with radiation to left arm or neck with feeling of congestion or discomfort in the chest. It also increases on exertion. Rare varied presentation of cardiac originated pain can occur.
In your case it appear as simple muscular pain due to spasm or twitching. A safe option is investigation to find the cause & rule out cardiac & confirm muscular pain is essential at this age.
Shoulder pain also needs x-ray as it can due to many reasons. In the mean time you should start regular wheel movement (exercise) of shoulder joint. Use topical pain relieving creams available over the counter.
Hope I have answered your question well to your satisfaction. Wish you speedy complete recovery at the earliest.