In simple terms, what happens is that undigested lactose passes into the large intestine where, in several different ways, it is broken down into glucose, which then is passed out of the body in the stool. So a finding of high glucose in stool is not surprising in someone who is lactose intolerant.
Definitely follow up with your doctor about this to make sure that nothing else is going on, and to see what if anything you should do about the lactose intolerance other than, obviously, avoid lactose.
Sorry, I'm pretty sure I've posted this in the wrong section.