I had the same problem with my 1 year old she wouldn't poo for days one stage she went 11 days and would cry when trying to poo so i started to give her a stool softener and change some things in her diet i cut out bread,pasta,carrots and even Bananas i added more fiber such as pears,apples and even grapes do wonders
No urine or fecal sample was taken? What kind of blood test did they do? Did they discuss the possibility of celiac disease or foid intolerances with you? Has your daughter been through allergy testing? This could be an ongoing UTI, parasitic infection, or immune disorder.
If you're in the US, my recommendation is to call the closest children's hospital for more help. Your family deserves better than the GIs you've worked with.