Mild chronic pancreatits, not coused by gallstones or alcohol abuse. I do not drink and no stones seen in an ultrasound. What could be the couse?

Idiopathic. That is s medical term for unknown cause. Many times the etiology of chronic recurrent pancreatitis is unknown. Also sometimes pancreatitis can be related to elevated blood triglyceride levels.
Varied. If you were unlucky enough to have had just one small stone that passed into your bile duct then that could have caused the pancreatitis and been passed so it remained undetected. Elevated triglyceride level, steroid use & drug interactions are other common causes. There can be genetic predisposition, viral infection, and cancer as unusual causes. You should discuss your concerns with your doctor.