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The description suggests that its a lymph
node. But only a clinical examination can confirm that.
Lymph nodes are small mass of tissues present all over the body, in presense of an infection, inflammation, or malignancy the become enlarged , visible , palpable.
The pain in you shoulder could be unrelated to this, as you mentioned about the heavy bags you have been carrying over the same shoulder.
Please visit a general physician , to make a confirmed diagnosis.
Meanwhile give your arm and sholder rest.