IBS can have a psychological cause, but can still cause real discomfort.
Coeliac disease can have stomached ache type symptoms. Coeliac often comes along with type 1 as it's an autoimmune disease, the body thinks it has gotta kill gluten, but can't so instead starts attacking bits of its own healthy gut. You can find coeliac antibodies or ask for a gut biopsy to see if the gut is being attacked.
How exactly did the GP decide "all ok" or that the stomach ache problem was for referral to the diabetic team?