A soft swelling below your elbow,which was present since few months may be a Lipoma
,a swelling arising from fat cells.
As it is a soft tissue
swelling arising from fat cells beneath the skin,X ray
will not show any bony abnormality.
Because of the trauma or pressure that might have occured while leaning,the skin over it got infected and inflammed.But,lipoma getting infected is not reported in literature.
Another possibility is Sebaceous cyst
.It is also soft,occur in places where hairs are present.It gets infected more commonly and increases in size because of Pus and inflammation.
So,for correct diagnosis please consult your surgeon and get it treated accordingly.
Wish you good health