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.
a bee in (one's) bonnet
1. An impulse to do something; a notion.
2. An obsession.