DD is 6, 7 in November. She's been solidly on the 9th centile from birth which I've had no concerns about.
However she's lately looking a LOT smaller than her peers and having measured her she is 107 cm and about 17-18 kg. I've just looked the centile charts up and if reading them right she is more like 0.4th centile.
Is this a worry and should I ask for a check up? She is healthy and energetic, enjoys gymnastics, swimming, ballet and winding up her brother. Achieving well at school, loves her teacher, many friends. We have had issues of more challenging behaviour at home which could ? be tiredness or unhappiness although Id put it down to being six and strong minded 
Appetite not great but not non existent either. She has had chronic constipation again from babyhood which I felt we were managing OK with Movicol at present. We had a paed referral about 18 months back which raised no significant issues.
So do I just have a happy healthy natural born gymnast or is there something I should be doing? Advice welcome. Thanks.