My arm aches like mad?

Every 2/3 weeks my arm seems to tense up and eventually go completely numb. It aches so bad i cant even bed it into any position as the pain is really bad. Its always (bar once) on my right arm at the top between my elbow and my shoulder. I do quite heavy lifting for a living but whilst i have been in my job for 2 years, this pain has only been around for 2 months. Painkillers usually reduce the ache/pain a little but it normally goes away fully after 24 hrs only to reappear 2 or so weeks later when i get the familar ' begginings of a dead arm - any help much appreciated - ps its not the way i sleep either