a bee in (one's) bonnet
An obsession, often with something that is strange or a source of agitation. Ever since the blizzard last year, dad has had a bee in his bonnet about moving to a warmer climate. It seems that Mike still has a bee in his bonnet over the criticism he got in the staff meeting.
bee in one's bonnet
A strange idea or notion; also, an idea that is harped on, an obsession. For example, Bill's got a bee in his bonnet about burglars; he's always imagining strange noises. This term, which replaced the earlier have bees in one's head, transfers the buzzing of a bee inside one's hat to a weird idea in one's head. [Second half of 1600s]