Old things don't give you rashes, allergies do. You are probably allergic to some synthetic or natural chemical in the fragrance. Just because the fragrance is old, or even bad, it would not give you a rash based on that fact alone. It has to be something else and I would bet on allergies.
I would find a reliable store that takes care of the tester bottles they have out, or a brand new tester bottle. You would have to do a side by side comparison to see what the differences are. If you know the scent well enough then you should be able to tell just from getting a whiff of something that has mutated. That is the only way I know.