Help please?

About a week ago, a small bump appeared on my cats head. It has since progressed into an open sore, and the hair has fallen of the spot. It doesn't appear to hurt her and she has been acting completely normal but it's obviously getting worse. We fear she may be scratching it so we put a cone on her neck to prevent her from scratching it.
She is an indoor cat who has not come into contact with any animals in over a year. Also I am pretty positive it's not fleas. I have attached a photo.

Does anyone have any ideas what this could be? Or any helpful tips for a speedy recovery?

We are soooo worried and just want her better.

P.s her name is Nala and she's three years old.

She really needs to see a vet to get a proper diagnosis and treatment plan. And, sooner rather than later since it is getting worse.

As far as speculating what it *could* be:
A form of dermatitis (skin allergy)
- allergies to be food or environmental and can set in suddenly despite any changes.
Eosinophilic Granuloma Complex
Abscess
Cancer (unlikely due to her age)

Now that you've put the cone on it, is it still getting worse?  I would guess that she was, indeed, scratching at it once in awhile, which would aggravate it.

As to what it is, there's really no way for us to tell.  Could be anything, as Catpack said.  I'm NOT thinking it's an allergy of any type or she'd be scratching at it a LOT (I'd suspect).  Abscess is certainly possible, bug bite is possible, but COULD be something worse, so I would definitely take her in for a check-up just to make sure.