I am sorry, I've been in a position where it was scary as hell wondering how I'd get the money for groceries, albeit this was a long time ago But...how can we know what is wrong? I can wonder aloud - is it a bug bite, a spider bite, is it an infected puncture wound, is there an abcess developping...but I'm not there, I can't see this, or feel it (i.e. an abcess is very firm to the touch and warm and you can feel depth to it if your cat allows you to even touch it as they tend to be very tender.)
I don't know that even one of the vet techs here would feel they could hazard a guess.
I am really sorry. My suggestion? Do some google searching on "cats abcess" and "cats spider bite" and see if you can find a vet article with photos, see if it matches what you are seeing.
If you can determine it's an abcess, folks here can share what care they've given their cats (though this can so quickly require draining at the vets and strong antibiotics) and how long it took their cats to recover etc.
We can share our experiences, but we really can not possibly diagnose your kitty (even if we were vets) by your question.
Please read this article on what to do when you can't afford a vet...and keep us posted as to what happens?click here