As asked in this thread by Mangetout, just a quick update. I am not asking for medical advice, I went to see my doctor this morning. 
It is an infection, Erypsilas, or Saint Anthony’s fire, and I got a ten-day antibiotic-treatment for it.
Glad you went to the doctor, dear. I was so hoping you would.
I’m glad to hear you’re getting treatment and I wish you a speedy and uncomplicated recovery.
Erypsilas? Wow. I was way off.
Pshaw, it can still lead to flesh-eating bacteria if untreated, according to Wiki. 
Thanks for the well-wishes, everyone. 
On the brighter side of things, at least it’s not a case of St. Elmo’s fire.
I had been confusing it with that, so I had all these weird images of being adrift at sea.