We are not doctors here, so we don't know all the possibilities. Although you are not feeling a lump or swelling, it is possible that there is some sort of swelling deep inside putting pressure on a nerve and causing pain. Breast cancer can show up in the armpit, so you need to be alert to that possibility. Whenever a reader reports pain on the left side, I always mention getting checked out for heart issues. Usually the guidelines talk about pain radiating down the left arm as being a potential heart problem, and your pain seems focused in the armpit, not in the chest or all the way down the arm, so the chances that this is heart-related seem small. Nevertheless, pain that has lasted for three months needs to be checked out by a doctor. There is a good chance that it will turn out to be something harmless like a muscle issue, but please see a doctor just to be sure.
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