It's possible to be a poison ivy rash though having a rash on one side is a common symptom of shingles. Apparently, anyone who has had chicken pox can get shingles (yikes! I had no idea....).
Have you found any poison ivy / oak in your yard? Any way you could have come in contact with it? It's true that the most likely contact would be forearms, legs, etc. The skin is thicker on the hands so may not show up there. I guess I'd be looking for that smoking gun - plants where you've been working lately just to help determine if its a possibility. Sorry - this isn't much of a conclusive answer. I did look at a few sites with shingles information to see about the similarities and many of the pictures I saw the 2 rashes did seem to resemble.
Anyone else with thoughts?