You know them better than anyone, and I think that, if you are seeing it, then it's true.
Itching can be absolutely anything, from pollen/grass issues to food/treat or laundry detergent issues.
In fact, Ds called me this morning about Thyri having trouble breathing through her nose, and having a rash on her cheek.
After a while of playing 21 questions, (she hasn't tried a new vegetable in over a week, etc.) he realized that they didn't have the detergent that he'd been using, when he went to the store, and he'd gotten a mild brand that he used to use. I was able to remind him that he'd stopped using that brand, because Thyri had stuffiness and rashes with it.
So, it's off to another store to get the last brand that he was using, and guess what? It's laundry day-groundhog day for him!
The problem is, it's on her clothes, her bedding, the towels that she uses after her bath, but also the clothes that they wear that she lays her face against, their bedding, etc.
So, it could be the same with the Chelsea. Have you changed detergent? Softener?
Have you changed treats or food, or added anything new to their diet?
It could be pollen, as I said, but also something else...has the gardener put anything on the grass?
As for Missy, starting stiff, then warming up to about normal, sounds like arthritis, doesn't it? Or, other joint problem. She's pretty young for that, but it happens. It would be worth having it looked into the next time that she has a vet appt.
In the meantime, does she take any joint supplements?
Prayers for both girls, and you as you try to figure things out. It's not easy!
Love All Life, Thank You For Posting! :)