Because scratching my arms consumes most of my thoughts right now, I was looking at poison-ivy.org to learn more about one of my ever-growing enemies. There are lots of pictures of poison ivy rashes and the ones that look most closely to mine (and what mine normally look like) have the following descriptions:

"The scratch suggests the person might have pushed past a vine and gotten a scratch. The scratch opens the skin, and lets the plant oil in much more directly."

"To get a rash like this, pull out poison ivy vines with bare arms and scratch your arms deeply as you pull the vines out. Once the skin is scratched, the oil can get right in a produces the distinctive scratch-rashes."

...hmmm, and here I thought the wind just had to blow in the wrong direction!