New 2cm node in armpit? 8mm ones in other arm, ? cause?

Often these are just reacting to skin irritation in the nearby region or some process further down the same arm. If it becomes painful or continues to enlarge a warm compress applied several times/d can improve blood flow and self-healing possibilities. If it begins to worry you, have it evaluated.

Can you be sure that these are lymph nodes and not skin lesions? If you stretch your arm up& outward, lymph nodes will recede deeper into the armpit while skin lesions, like ingrown hairs or inclusion cysts remain palpable and visible. A physician can examine and advise you further on diagnosis, testing and follow-up.