the.com/bee in bonnet

means To have a fixed, often obsessive idea or preoccupation that you keep returning to, usually one others find a bit much.

from From the old image of an actual bee trapped buzzing under a woman's bonnet — imagine the frantic, distracted agitation that would cause, unable to think of anything else until it's dealt with. The phrase appears in Scots and English usage by the 18th century, often as 'to have a head full of bees,' before settling into the bonnet version we use now. The French have a cousin idiom too, suggesting the buzzing-brain image was simply too good for one language to keep to itself.