Cooties

But it was misdiagnosed.

Itching began last Wednesday.  Rash started Thursday and kept getting worse.  Doctor visit on Friday with a diagnosis of mites (from cuddling the cat on Tuesday and Wednesday) or chiggers (from sitting in the grass on Tuesday) or scabies (huh?).  This [substitute] doctor didn't test because these problems are all treated with the same Rx-cream and the same washing regimen.

Followed instructions carefully.
Refrained from hugging and touching anybody at the rehearsal dinner.  I told them I had cooties and to stay away for their own protection.
Wore long sleeves and long skirt and high neckline for the wedding, continuing to be careful to not-hug or at least to avoid any skin contact. 
Rash continued to get worse.
Continued to wash, re-wash, and again re-wash sheets, towels, clothes, etc.
Itching kept me up at night.
Rash continued to spread.
Sores became oozy and weepy.

Returned to doctor.  This time it was past office-hours and we went to FastCare.  This doctor diagnosed a bad case of poison ivy.  She prescribed steroids and an astringent wash for the sores.  In two hours I could already see significant improvement!  I slept comfortably last night.  Sores are healing nicely and no longer spreading.

The chigger diagnosis not only delayed treatment for the poison ivy; the medicine for it also worsened the rash and spread the urushiol oil.  It's sad that I could've felt this good for the wedding if the rash had been correctly diagnosed the first time. But at least it's on the mend now.