I was thinking exactly what Sequoia just wrote. I'm not sure where you mean exactly by the back of the head, but my head-neck attachment area back there is an area where I sometimes feel pain and stiffness, almost like the area is swollen.
I still would definitely get it checked out, but my amateur guess is that it is inflammation and not something more worrisome.
I hope you feel better quick!
Female, age 47, B-27 Neg. USpA, Sjogren's, EDS III, Fibromyalgia, etc. etc..
Meds: Actemra, Mtx inj., Prednisone 2 mg (trying to taper off), Celexa, Lyrica, Folic Acid, Calcium, Magnesium, Vit. D. Diet: Dairy free, Wheat free, Potato free, low starch.