I went through a period of brain fog after losing my husband so it certainly isnt uncommon and as Dave says,its as though the brain does go into some kind of shock for the first few weeks, we know what has happened but we are also organising things, my theory is, that when we are doing that its like we are still caring for them,and when its done suddely we dont know how to cope with not having anything more to do - and thats when we start to really grieve, and the brain fog is somehow trying to protect us by only letting a little bit of what our emotions are doing in at a time.Physically our emotions are an intrinsic part of how our brain functions and to suddenly be hit by so many and such extreme ones, there is probably something like a shut down valve operating. I distinctly do remeber setting off to walk to work and actually seeing a mist, maybe a real one or maybe not but through the mist people were going about their normal day and i wondered just for a moment if it was me who had died.
As Dave said there is no harming checking it out with your GP though if nothing else to reassure yourself, it could be something hormonal thats going on as well and hiding under the grief, a blood test would probably find out.