What's wrong with my dog?

This is the first time I have seen this but she could have done it before, while eating rawhide that she was chewing on her eyes( a white part from the side and bottom) were popping out of different directions out of the eye socket. It was very scary and it stopped when she stopped chewing otherwise she is normal and I have never seen her do this. Could this be a symptom of some sort of brain thing like a tumor? Is it a symptom? We are taking her to the vet but I just hope it is nothing serious because I have never seen this before and I am really worried, my last dog died of a brain tumor and I don't want that to happen to my dog