I think you have the exact thing i have right now!! And it always starts when it starts freezing... Not just getting cold but freezing. and it comes in waves also, I'll be just sitting around and bam i can feel my face and head in pressure and the brain fog comes right after...
This happened about 3 years ago and it was with a sinus infection.. It took awhile for the infection to finally get bad but once it did and I got rid of it, i don't think i experienced the head fog anymore.
I don't know how to help as I am in the same exact situation you are..LOL, but hopefully this will pass soon..