Whats this swelling in my gum/cheek?

[kaydawg]

Yesterday as soon as I woke up I had a deep dull pain between my lower back molar and my ear. It was intermittent all day. I assumed it was TMJ and didn't fuss.
Today I woke up and the pain was gone but I've got this weird swelling on the gum behind my back molar, and my cheek is puffy???
It doesn't hurt a lot but I'm really scared it might be an infection?
Obviously it's a bank holiday so I can't see a dentist for at least 4 days.

EDIT: I will add I was probably too aggressive when I brushed it yesterday.

(the back tooth is quite stained and I need to book a scaling)

 

[MattKW]

Looks just like cheek fat to me. It isn't helped by the outward tilt of your 2nd molar, but that's just your anatomy. Other possibility could be the presence of an unerupted wisdom tooth pushing the 2nd molar outwards. Not urgent.