Okay girl, first off you are going to take a deep breath and try to relax. You spacing out right now is not going to help your Marshmallow.
I am no vet, but from everything that I have read, it does sound like an abcess. Please do not think of yourself as a bad meowmie or feel guilty for not noticing this sooner. With a cat, an abcess can pop up in a matter of hours and yes, they can become huge. We had one stray cat that had one that I swear, could fit in the palm of my hand that was on part of his face and neck.
Marshmallow will be okay until you can get her to the vet tomorrow. If she is no different than what she normally is, I wouldn't worry about it too much. I know that it is easier said than done, but I have had to deal with a few cats that have had an abcess and they were fine until I got them to a vet.

So please, try to get some sleep and let us know what your vet has to say.

I think you need some PT HUGS right now...................so here's a biggy for you.

((((((((((HUGS))))))))))