What Causes Pain In The Legs And Lower Back Of An Infant?

Hi question is about my 23 month old daughter she has complained about legs n lower back/bum area hurting her since she was about 12 months (walking from 8month old) it started to happen about once a week and has gotten to be every couple of days sometime its every night where she is crying and saying its hurting this lasts for 30 mins to an hour she twitches during this time and she can t walk when she s like this and won t let me touch her legs or back been referred to hospital (seen in may) been to doctors numerous times I get told to wait till her appointment comes