What is this rash?!?!?!

I have an appointment with a vet for my newly adopted kitties but it's not for another week and a half... I was wondering if Sally's rash looks familiar to anyone?!

just some info, Max her brother licks her ears a lot it seems... it could be how affectionate he is(super cuddly, licks me or anyone cuddling him) she's also very sweet but not as openly affectionate or as used to me and my son yet... but she IS addicted to temptations... i discovered this because she's vocal but even moreso in the kitchen... i bought her some just to wean her off her addiction and help her with the transition... both cats are overweight and clearly were spoiled in all the wrong ways when it comes to nutrition(and max has been declawed because he apparently is(was) a furniture killer)... *eyeroll*

anyway...

I've only had them a week and noticed the rash 2 days ago...

here it is:

My first guess would be flea bites.  Have they been treated for fleas?
 

That's what I was thinking it might be, considering max has no sign of rash and they told me when I left that he was treated and she wasn't because she wouldn't take the treatment. I'm just worried about her because I don't know her well enough to know if she's acting strangely or really unhappy/ uncomfortable.

Is she scratching? My first thought would be fleas or a mild allergic reaction to something in the environment. Is she uncomfortable or licking herself excessively?

Is the rash just where she was shaved?  I once had a cat that had razor rash after surgery.

Have you combed her to check for fleas?  You need a fine tooth comb to do this, and you stand her on white paper.  If you get any black dirt out, wet it.  If it stains red that is flea feaces and you need to treat all your pets.

She wasn't shaved! She was spayed years ago, they're 5 and a half!
I think she licked/chewed off the fur. I'm thinking it's flea dermatitis, her brother was treated at the SPCA but she wasn't because she wouldn't take it. I don't really think she has fleas though because she has no flea dirt on her anywhere. I just think she probably ha a reaction from getting bitten at the shelter. Thankfully they're still under the 30 days per insurance the shelter gives you so any treatment she'll need will be covered I just want her to feel happy and comfy with us. Her brother is right at home and loves to curl up in my arms for snuggles and plays with my son which is freaking adorable! Lol
Sally is warming up to me but runs from my son Jo follows me every time I go to love on her.

Oh and no she isn't scratching but licking a lot