Research endometriosis on google it sounds like what you have and normal tests won't show it you need to see a gyno
Low abdominal area across pelvis contains quite a bit of the small intestine, and the area that runs up the midline toward the bellybutton is suggestive of the sigmoid colon. So have bowel spasms been ruled out? Any anti-spasm meds tried? When the pain hits, do her bowel habit change - or is there any change in bowel habits? Has she been on any antibiotics in the last year?
Midline spasm can feel like someone put a huge clamp on the sigmoid area squeezed - makes you want to press on it, curl up and feel like dying.
Any bloating or gas?
Aches and pains? Where? Anything specific? Tingling/numbness, or? Brain 'fog' or fatigue that hits like a ton of bricks?
If there are no intolerance issues in the family, have you tracked her food intake specifically and checked for correlations? There may not be a familial link, but intolerance could raise its 'ugly' head anyway.
Is she on any meds?
Low abdo pain, across pelvic area,mainly, can spread towards tummy button, liver and pancreas tested. No food intolerance problems or AI problems in family. Tired, aches and pains, depression. Cycle is regular 28 days, any clues?
Where specifically is the pain located? Have you kept a food diary to see if any types of food seem to trigger the pain? Did the blood tests look at liver and pancreatic enzyme levels? Any food intolerance issues in the family? Any other symtpoms that might not seem to be associated with the problem - but pop up? Any autoimmune issues in family?