Brain fog for the past couple of weeks coupled with dull ache behind both eyes. stiff neck also. my lexapro (escitalopram) dose loosing effectiveness? a migraine?

Not a migraine: Could be a cervicogenic headache which is from neck muscles in spasm. You could try a massage of the area or a hot shower playing the spray over the neck area while stretching out the neck muscles. Moist heat or ice packs over the sore neck areas could actually make your head feel better. The brain fog could be a neurological issue. Good luck.