2yo suddenly shy and clingy

[Twixed]

In the last few weeks dd has become very reluctant to join in at toddler group and such like, will just cling on to me, turn her face into my legs etc. Even drop offs at nursery have changed - she used to run off with barely a second glance. She will go off after a few minutes and there are no tears but I'm wondering why the change.
She has a newish baby brother, whom she adores, but he's nearly 5 months now so I don't think that could be it. She's her normal boisterous self at home, park, shops etc.
Should I be concerned or is this a normal developmental stage which others have experienced? Any recommendations for reading up on this/ how to handle? Apparently my dh was very shy as a youngster, not that you'd believe it now!
Thanks

[exoticfruits]

Probably just a phase. Don't draw attention to it.