Please dont get too het up about graphs and percentiles and statistics.
I did. My daughter on her first birthday wore clothes aged 0 - 3 months and seemed quite odd toddling around in such. Paediatrics also wrote 'failure to thrive' on her notes, which greatly upset me as to me she was thriving fine. Alert, bright, walking, etc, did not suggest failure to thrive to me.
Your daughter has a balanced diet. You can do no more. Some of us are just designed to be petite or small built. I dont know much about the constipation except the obvious - fruit and water, but I didn't want you to be worried or upset or too concerned about her weight. My grandma told me to look around the school playground and see the children of all differing heights, sizes, etc. Family genetics plays a part too.
I was a first time mum - ready to listen to professionals. Common sense from grandma prevailed. I think there's too much paperwork, recording on graphs these days. If your daughter is reaching development milestones, eating a varied balanced diet, she'll be fine.
My daughter is now 21, weighs 7 stone and at 5ft 1 is very proportionate. No need for all the fuss and labelling.