Cellulitis

GP or A & E asap. You need antibiotics.

Thanks.
Seriously, I do mean that.

Does it spontaneously just happen, would you know?

I can't see any breaks in the skin, but this one leg has been swollen for a while.

Doctor or even A&E
It needs a/b’s, possibly IV depending on your age, health and the seriousness.
I was seen within two hours when I contacted my GP online with a photo of the cellulitis on my face.
Just to cheer you up, the 51 year old daughter of one of my closest friends died of sepsis about a month ago, she had resisted going into hospital (as advised) with Cellulitis as she had no one to look after her elderly poorly cat. Eventually taken in by ambulance and died less than a week later.
See a doctor- not just a phone call, send them a picture.