What Causes Shoulders And Arms Pain With Stiffness?

Hi, I am worried about Computer causing cancer. I have developed pain in my shoulders and arms and possibly under my armpit on the left although I maybe do not think that there is pain from the lymph nodes but it appears that way sometimes.. This has been now 4 months with stiffness and pain. And now it appears to be developing the same symptoms in exactly the same spot/places on the Rt side I have seen my Dr and he referred me to a physiotherapist but the pain does not seem to ease. I have no swelling. Am I being over anxious? And could this be related to posture whilst at the computer? Thanks for your time.