This often happens.  Pharmacies now have out two kind of medication.  The
first is a preventative (Imunoak, etc.) that increases your tolerance level for
the poison oak (ivy, sumac) oil by exposing you to small doses gradually.
Sometimes it works or helps, but sometimes it doesn't.  The second type is a very
caustic and abrasive remover of layers of the epidermis, useful if you use it
right away, but I imagine that would only worsen your situation.

 There is no substitute for scrubbing with Fels Naptha soap immediately after
exposure to remove oils before they suppurate don't try to treat it at home
with calamine lotion.  I nearly had a gangrenous wrist that way and healing
took forever.  See a doctor for cortisone shots. I also experienced a resistance
to poison oak as a child (son of a botanist), but got it bad when I was 20 and
have done so ever since.  Must be age!

Bob Hoover