Find a dermatome map on Google or Firefox. If it's Shingles, it's along your C6-C7 dermatome route. I have seen 3 patients with Shingles recently, and the pain and damage can be lessened with antiviral medication. Shingles can last for several months and leave permanent pain and scars.
If it's identical on both sides/arms and showing up on your lower back, palms and feet, it could be DH, a sign of Celiac disease. A dermatologist could tell you in the office without too much trouble whether it's DH.
These things both tend to crop up as weather changes. Both are usually related to stress, pain, trauma and anxiety, too. As if some type of trauma weren't enough on its own.......
If its topical, maybe you rubbed your skin on detergent, bleach, chemicals that burned?