It could be irritant contact dermatitis or possibly allergic irritant contact dermatitis.

In the first case it could have been from just the wool hat rubbing on the skin possibly due to something like a detergent residue in the hat. The fact that you have never experienced it from the hat when you've worn it before might not matter as your body is constantly changing. That which was never a problem before might be one now. And, if you wear the same hat a month from now you might not have a reaction.

In the second case, it could be an allergic reaction to something like poison ivy, poison oak, or something similar. I sort of doubt that though because where you live those irritants aren't in bloom yet (if they even grow in Idaho). Did you eat any seafood a day or so before it appeared, especially shell fish? That's another possibility.

My guess would be that if you went to a dermatologist and he couldn't diagnose it specifically, he'd just call it irritant contact dermatitis. It's sort of like their catch all diagnosis. Try some Benadryl cream and see if it goes away in a day or two. The Benadryl cream is an antihistamine and should clear up the symptoms if the condition isn't something more serious.

Jeez, it almost sounds like I know what I'm talking about!

No, I'm not a doctor, but I did stay at a Holiday Inn Express last night....