Ok the good news is that no surgery is needed for the time being anyway.
The vet thinks the lump is softer and not so solid and a bit more moveable.
He drained off a small amount of fluid with a needle to see what was inside

(couldn't do this last week as it was so solid) and looks like a mixture of blood and pus

, so the conclusion is that it is probably an abcess. No idea how or where it came from but def not teeth or gum related so that is good.
My little baby was so good with the needle and didn't murmur

Extended anti-biotics for 5 more days and hopefully that will be an end to it
Thank you all for your kind wishes and words, it means a lot when you are worried about you little furbabies