I Have A Rash - Started About 2 Weeks Ago

I have a rash - started about 2 weeks ago on my right shoulder above the armpit - came on suddenly very itchy, many blisters filled with cear oozing fluid - I am very allergic to poison ivy and thought this was it - except that it is January and I have been out with coats on - seemed unlikely. After about a week the blisters dried out and the skin was no longer itchy - but I developed additonal areas on my forearms with a light red, very small blisters and very itchy as well. Now after 2 weeks it seems to be going away. Never got it on my trunk area or legs/face. Am treating it orally with aspirin and Allegra and topical antihistamine seems to help. My doctor did not have a good explanation - said it was not shingles. No flue or fever or really any pain. I would like to try to determine the cause so that I can avoid exposure in the future. Also can not think of any new detergent or soap or unusual activity that would have caused this. I am fair skined and prone to dry skin and rashes. Any thoughts?
THank you!