I have the exact same reaction. I believe it to be from over-exposure to the glands on the plant.
My buddy thought I was kidding when I told him this, and tossed a keybump full of kief (sp?) on my forearm. Well, I scratched it to hell and got out of my job for a week with "poison ivy". - Leads me to believe its just the glands or crystals
With me, it didn't develop until after I had handled cannabis extensively. My buddy, who knows a LOT of people in the game, said he knows several people that use long sleeves and kitchen gloves when handling due to the same reaction.
I have found this out about treatment...
There is no cream/ointment....
Once you begin to feel the itch coming, and believe me, you will; do NOT, I repeat, DO NOT TOUCH ANYWHERE NEAR IT.
After about 10-15 minutes mine go away completely. However, if I scratch it even once, the reaction spreads and the bumps grow in size dramatically as well as lasting for a day at least.
Oh yea, the reaction is even more present and potent with marijuana that is still alive...
