bee in bonnet
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 single idea or a thought that remains in one's mind; an obsession. (*Typically: get ~; have ~; give one ~.) I have a bee in my bonnet over that cool new car I saw, and I can't stop thinking about it. I got a bee in my bonnet about swimming. I just wanted to go swimming all the time.