One idiom that could be used jocularly is
- She's away with the fairies.
Urban Dictionary allows for the temporary abstracted state:
The expression means to be in a daydream, to be in one's own world,
sometimes to be deep in thought and having one's gaze unfocused.
But care has to be exercised; it is often rather cruelly used for a more serious, lasting, condition:
- away with the fairies [idiomatic]:
[1] Not all there; slightly crazy.
...
[Wiktionary]
- away with the fairies
[informal]:
out of touch with reality
[Collins]
Apparently of Irish-English origin, the phrase away with the fairies means giving the impression of being mad,
distracted, or in a dreamworld.
[Pascal Tréguer; Word Histories]
When you hear the expression away with the fairies, you imagine someone rather useless. Someone chronically
distracted, with a short attention span and no common sense. Someone –
if we’re not being polite – a bit batty.
[Zoë Marriott; RLF]
......................
Probably safer is 'She's in a dream' (not 'in a dream world'):
be in a dream [UK]:
B2. to not notice things that are around you because you are thinking
about something else.
[Cambridge Dictionary]
I'd say there is more a hint of euphoria than of apprehension, or even of just everyday distraction.