As per your explanation I understand you have lymph node swelling under your arm and below the breast area towards later side.
Here I suggest you need to understand
lymphatic system is the one which carries waste metabolic end product from different part of our body to
kidney and heart to get it filter here. And this lymphatic chennals are present all over the body even in our fingers too.
lymph node is an oval-shaped organ of the lymphatic system, distributed widely throughout the body including the armpit and stomach and linked by lymphatic vessels. Lymph nodes are major sites of B, T, and other immune cells. Lymph nodes are important for the proper functioning of the immune system, acting as filters for foreign particles and cancer cells. Lymph nodes do not deal with toxicity, which is primarily dealt with by the
liver and kidneys.
If you feel there is a swelling in lymohnode which is increasing and getting tender day by day then I suggest do not wait and visit your gynec immediately along with a vascular surgeon to for having better idea on your issue. My aim is not to panic you but if there is any thing related to cancer then early detection and treatment will help you to get cure easily.
And
neck pain can be possible with this. So I suggest first you need to evaluate on lymph node swelling and take treatment for it. Later still if neck pain continue you can visit ortho and physio for further advice and treatment.
Take care.