What Causes Pain In The Elbow And Arm?

Hi there, just today i have had pain from my right elbow to my armpit. It seems to run underneath my upper right arm. It is very tender to touch from my elbow up to my armpit, particularily around my elbow. Its appears to be tender along a line from these points. It feels like there is an inflammed tendon or something. It becomes taught when i outstretch my arm between these points. It appears to tighten up if i rest my arm and is quite sore then when i outstretch it afetr resting it. There is no visible swelling or redness. Im a little bit worried. Im 39 years old. Any ideas would be appreciated. Thanks, Ray.