3.5 year old suddenly taking ages to fall asleep

[Changedup]

Until about a week ago dd would fall asleep within minutes after going to bed between 6.45 and 7. Now all of a sudden she isn't settling til 8 or later and I can't work out why! Nothing has changed. She hasn't napped for almost a year, she has the same room, blackout blinds, routine etc. I just can't work out what's going on. She's not waking up any later to compensate for going to sleep later either.
Does anyone have any ideas on why or what I could do?

Maybe she doesn't need as much sleep. Why not try a later bedtime

Thanks. The thing is she does seem tired at different points through the day and until very recently was asleep b4 I left the room sometimes as early as 6.30. It's just odd that in the space of a week or so she is now taking 1.5 hrs longer to fall asleep. That's a big sleep decrease almost overnight!