Greetings.  Well as per your description.... Madam the swelling need not be from skin alone and there are situations when a swelling forms below the arm pit region. I would be happy to guide you  further.  It could be something thats should be checked and treated at the earliest. You can contact us  at Health Cottage Hospital, Bangalore. REGARD & GOOD LUCK.

Thanks for the query. This presentation can be seen with many diseases. This is limited information to make precise recommendations. The best option is to consult a Dermatologist to understand the condition. A thorough evaluation if necessary should be done. Hope it helps you.