This is an all too common injury among those who work in fabric stores and lift bolts of fabric with a handgrip on their ends. After a fellow shop owner spent most of a year in a sling, I began to forbid my staff to lift more than one bolt at a time that way. We used to bend at the knee and "hug" the bolts.
Is it possible you are lifting something or using your arm repetively in this manner?
Jan in VA