My 5yo DD has been doing this for a month or so. We had about a week of real anxiety, full on sobbing tears about not being able to go to sleep despite obvious signs of tiredness. She worked herself into a right state about it, despite us never shouting at her about it, and trying to help her.
I think I made it worse inadvertently as about a month ago I promised her something as a reward after a week of good bedtimes, so perhaps she was anxious she wouldn't get the reward if she couldn't get to sleep.... Poor mite.
We started sitting with her for a bit, she would like us to sit with her until she falls asleep, and rub her back, but I also have a BF baby and tbh I want a bit of time to myself in the evening!
I now do normal bedtime, read her a story, then go in again after feeding DS to sleep, then after a short chat or making up a little story I tell her I want a cup of tea but that as soon as I've finished the tea I'll come back and check on her.
Miraculously this has worked and we haven't had tears for about a week, although it still takes her ages to actually fall asleep, often still awake after 9.
I think it's lack of stimulation after finishing school, less physical activity than normal, and I think we're eating later than normal too.
Desperately trying to rejig our timings.
Hope you manage to come out the other side too. It's so frustrating, not their fault but not much parents can actively do apart from be there and make them feel reassured and loved.
Good luck 