Usually when a client says the eyes are rolled back in the head what they are actually seeing are the third eyelids raised up over part of the eye. This is the inner eyelid that one normally wouldn't see. It helps to protect the eye and also to spread tears across the eyes surface. When it is raised it can be because the eyes are irritated, there is injury to the eye or possibly from fever. In some cases the eye can be dry and the eyelid stick up for a short time but there is no actual heal issue. What I would suggest you do is monitor her and her eyes for any changes. If the eyelids seem to remain more often or she is squinting or seems painful then I suggest a trip to the vet tomorrow for an eye exam.
If it isn't the inner eyelid then it is possible that your dog had a seizure. Other things you may have noticed is tremoring, loss of coordination, and loss of bowel or bladder control. With one seizure most don't rush into the emergency vet. Again I would suggest monitoring and this week try to get your dog in for bloodwork to see if the vet can isolate a cause. In most cases with young dogs seizures are consider idiopathic.
It is possible that the bloodwork will give you some answers but more times then not it doesn't and they seizures are considered idopathic. However, I will mention that the older the pet gets the more likely the cause of a seizure is a brain tumor when there is no other cause like organ disease or toxin.
Please let me know how she continues to do. It is possible that your vet may suggest a CT or MRI or even start her on medication.
Ask your vet about Cushing's disease and hypothyroidism as a possible cause for the seizure to occur.