Hi,
Welcome to the ehealthforum and I am really glad to help you out. A lymph node is a round organ of immune system & is connected with the lymphatic system. It gets enlarged if thereâs any infection in the vicinity.
They tend to entrap the culprit organisms and prevent the spread of the infections to the blood. It could due to an infection in your arm or in the armpit. Allergy to the deodorant can also produce this lymph node enlargement. You definitely need to consult a physician as you might require some antibiotics to take care of the infection and some anti-inflammatories to take care of the pain & inflammation.
I sincerely hope this information helps. Take care.