Hopefully someone with more experience will chime in, but I'm in PA too, and I was noticing one of my very tame (almost obnoxious) wilds had thin areas. She gets right in my face so I can see very closely. It wasn't as drastic as your girl though. In about a week she had grown in the most beautiful silvery coat, with the red-brown highlights on back, top of feet, etc. Her mother has kept the same almost agouti-looking grey, but her coat's starting to look a bit thin now. If you haven't added any new branches that could have mites or something and she's not itchy I'd guess she's getting ready for winter. I guess just keep an eye on it and check back if something changes (gets worse, shows discomfort/irritation, redness, etc.)?
How do you keep windows open 24/7? THAT'S dedication! Are you overwintering?