It all started about a month ago when I recovered from flu and the same time my birch pollen allergy kicked in. I got realy weird feeling in my head and at first I could't concentrate. Later I started to forget what was I thinking when something disturbed me for 2s and I started to have problems remembering thing like what I was doing yesterday. It though it was because of the flu and it would go away after few days, but it didn't. I started browsing internet and Brain fog perfectly describes how I fell. It is possible that seasonal allergies such as birch pollen allergy causes brain fog? It is already May and it doesn't seems to get batter. And it also weird because I suffer from allergy 3 years and it didn't happend before.