Hi. Thanks for your query. Noted your history of sudden pain and swelling in the inner part of upper arm, occurred without any obvious injury, is of the size of a jelly bean, and the pain increase on using the arm. The most probable cause in such a case is a small abscess from a boil. There would also be redness and local tenderness. I would advise you to consult a Doctor as you need a clinical evaluation, examination and most important the prescription for an antibiotic and anti-inflammatory and painkiller medicines.