I was bitten by something under my jeans a couple of weeks ago.
Came up with a red welt maybe four inches across, red and angry, hard and hot. Went to the docs and got antibiotics and a steroid cream in addition to the antihistamines and anti-inflammatory I was already taking.
Before it started to calm down, it got to 22cm across. I woke up the next night freezing and shivering, put a t-shirt on (when I managed to move) and woke up again an hour later boiling hot and wet through. Felt like I had flu the next morning. Fun.
The annoyance was I’d been bitten to buggery in France the week before and didn’t have anything more then pimples. I guess that compromised my immune system? Who knows. Bastards.