I tell ya, I been through the wringer with this little gal Luna

Now when I was brushing her tonight, I noticed a flaky patch on her back

She has a vet appointment on Saturday for her second round of shots (we waited b/c she was so ill), so I'm not sure if I should wait until then?
She's not on any flea preventative yet, the vet wanted to wait until she was all better so if she had any reaction to the stuff we'd be able to tell. She doesn't itch there at all. I did get her groomed almost 2 weeks ago, but if it was a reaction to something that they used, wouldn't it have shown up sooner? Oh gosh, I hope it's not from the Cowboy Magic, I love that stuff!
Any thoughts?