Are you based in the UK
@DP1117 ? Is so, are you anywhere near any of the vets on our vet locator?
Recommended Guinea Pig Vets
I can't diagnose , but from you are describing , it sounds like your guinea has an abscess and I really feel a vet check would help to see if your guinea needs antibiotics... As people have said above, it'll need flushing and the vet will show you how to do this.
I fully understand why you didn't want to go ahead with expensive treatment - BUT I'm really surprised your vet didn't offer a review appointment - to monitor and see if it was an abscess . Also, if it turns out it is a tumour , your guinea may benefit form pain medication .