Good call by Bruce there. If you have developed an intolerance for PG that you are ingesting, just the normal movement on your skin by the button could cause it. I would definitely consider getting allergy tested though.
All allergies really are are things (usually "normal" things) that our immune system has suddenly started seeing as an intruder. When our metabolism changes, so can that. I am still puzzled by the button somewhat.
Just an idea that I would try (which may be crazy as I will try a lot of things

) would be to sanitize the button cap and the area you are wanting to "test" on. Like the back of your hand or a inside forearm. After cleaning and without touching the two to anything else, repeat your button test.
If the rash doesn't reappear, you are likely dealing with a transference issue. If it does, you are most likely dealing with something ingested. The only benefit of that at this point would be to have a clearer picture of what your doctor will want to know. No offense to the medical profession but the less you know going in usually equates to that much less in your bank account afterwards.
