Hello Good Evening,
According to your description, you seems to have a Lipoma at that site, which is a benign tumor
composed of adipose tissue
It is the most common form of soft tissue tumor
are soft to the touch, usually movable, and are generally painless. But you find sore, possibly because of it's site.
In the management part you should go for X-ray and CT scan for provisional diagnosis followed by Excision and Biopsy to confirm diagnosis and definitive treatment.