Has she had any blood tests eg for coeliac disease, and also stool samples checked? There is a calprotectin test that checks for inflammation indicative of IBD.
I understand how frustrating and worrying it is when doctors confidently diagnose functional tummy pain with no solutions or any actual investigation.
My DD has had crippling abdo pain since catching covid earlier in the year. It was initially intermittent but following a vomiting virus a few months later the pain became constant and she’d often drop to her knees in pain even when having fun with friends on holiday so I found it difficult being fobbed off with the “anxiety” card.. Though it’s made me bloody anxious seeing her in pain all the time and not getting answers.
One specialist mentioned stomach migraines and suggested cutting out dairy, gluten, chocolate and citrus to see if that helped. He also recommended the name of medication to try. (He wasn’t my doctor, just offered advice over phone).
Another suggested cutting out eggs, dairy, soya, and gluten.
A gastro specialist thinks it’s very severe IBS, constant spasms triggered by Covid. He prescribed the meds the first doctor recommended. She’s only been on them a few weeks so can’t say if they are working yet.
A naturopath who I’ve also seen said lots of children that are constipated go undiagnosed and it will only be picked up on an X-ray. So we are also giving natural remedies for this. She also said most of us are lactose intolerant and suggested dairy free.
It’s also worth worming her (and the whole family) as that can also cause severe tummy pain.
We aren’t actually any further forward yet, despite all of the above, but it might give you some ideas! Good luck. And don’t be fobbed off if you are concerned x