What Causes Pain Or Tenderness In The Elbow?

For approximately 4-6 weeks I have been having pain/tenderness in my left inner arm from my elbow to my underarm when I reach, esp. overhead. Recently, the pain has extended to include below the elbow. I can no longer straighten my arm all the way when I reach overhead. This is different than past experience w/ tendonitis (elbow bone itself is not tender to touch). When I reach, there is a distinct pulling sensation. At times I can see what seems to be a ligament of some kind stand out a bit and with my other hand I can follow the line from my armpit to my elbow and into my forearm (tender to touch). I can relate no specific injury to this. I am a package handler at FedEx but have no pain what-so-ever when I work. I am a 56-y-o female in good physical condition; I build stone walls and do manual labor in the summer and am accustomed to sore muscles, joints, aches and pains. This is different. Was an athlete in my youth; no fam Hx of heart disease. Thanks for your help.