Could be a sign of maladaptive daydreaming. It's not typically diagnosed but more like a label (fairly new 'mental illness')
It's heavily linked to intrusive thoughts. If you spend a large portion of your day daydreaming about whatever, and it either affects your daily life (unable to concentrate on school work or unable to sleep because of your daydreaming) then it can be considered maladaptive, or if you're having intrusive thoughts that you keep going back to. Like, for example. If the day dream comes to being abused, you try to veer off that path and change the daydream to something else but you keep going back to it.
But as above said, it can also be intrusive thoughts if you don't tend to daydream as much as a maladaptive daydreamer.
Would definitely recommend seeking psychiatric help though, as these thoughts can be very distressing.