What Causes Shoulder And Arm Pain With Hand Numbness?

Hello, I have developed severe pain in my left shoulder and upper arm over the last few days, the pain was getting worse and was compounded by pins and needles down my arm to below the elbow. I have this morning developed a sensation of coldness in the arm as well and my hand is starting to feel numb. I work out at the gym regularly and most days carry a heavy backpack (about 12 kg, I weigh 52kg) for work. Is this a compressed nerve, and what should I do about it? Should I avoid arm exercises at the gym, for the moment? Thank you!!