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"She must have had a bad dream and crawled into bed with my mother."
A few words I think you may be having trouble with:
- vast = (in this context) probably, surely
- naar = (in this context) bad, unpleasant, creepy
-> 'naar gedroomd' literally means something like 'dreamed unpleasantly'
- gekropen = kruipen, kroop, gekropen: to crawl
Does this make the sentence understandable?