Hello and thank you for posting your health query on E health Forum.
Extensive testing which have been negative so far, indicates the absence of any organic reason for your abdominal cramps.
In absence of major symptoms like fever, loss of weight, appetite, bloody stools, etc, your abdominal cramps could be due to irritable bowel syndrome (IBS).
Additional history about your dietary habits and complete symptomatology can be useful in diagnosing IBS in your case.
You can consult a gastroenterologist for further advice and treatment.
I hope this helps.