People have varying degrees of sensitivity. If it's been that long, I'd see your doctor asap. I used to be immune but then got a mega-dose while taking a group of kids on a camping trip. Little chumps picked thick mature ivy vines and chopped it up and tossed it on the fire I'd built at the campground. It was hot, so I had my shirt off. I ended up coated with poison ivy everywhere that wasn't covered with my shorts. Took two rounds of prednisone to get rid of it.
http://www.rei.com/brand/Tecnu
Tecnu works for me for lesser cases.