So ... it turns out I have shingles →[More:]I had what I thought was a mosquito bite on my stomach. But it didn't respond to an anti-histamine lotion, it spread and blistered and hurt like hell.
After some Googling I took myself off to the doctor who confirmed that it is, indeed, shingles (the adult version of chickenpox). It's very painful, but hopefully the antiviral drugs I've been given and plenty of rest will sort it out.