Just a shot in the dark here but you said she scratches the back of her head and neck. Does she scratch around her ears? Could it be ear mites?

I noticed all three of my adults started scratching their head more this summer but they don't have fleas or ear mites so I've chalked it up to allergies too since it just started this summer and ended a few weeks ago but I do think it's odd that all three would do it at the same time.