Kate is this the same sleeve you've been using since August? And have you changed anything about the way you care for it? (I'm thinking a different product for cleaning it, a different product for cleaning you, etc....) August to now seems like a very long time for an allergy to the sleeve itself to develop, especially if it's been the same sleeve all along.
As you've probably gathered from my questions, I've had far more problems with soaps and lotions than I've ever had with a liner or sleeve. However, I have had reactions to what was "the same sleeve as always" if an old sleeve was replaced by a new sleeve of the same exact make, model, size and design. It was eventually determined that the manufacturer had changed something in the oil they used to lubricate the sleeve...once I started washing the sleeve very, very thoroughly before putting it on for the first time, the rash went away.
I hope someone with experience of actually developing an allergy to the sleeve itself checks in with something about their experiences that might help you out. In the meantime, good luck!