When I have relatives, who have already passed, visit me in a dream it almost always means the passing of a diferent member of the family. However, they never let on exactly which member that might be. And I don't really dream about my past pets but often feel that they always very close by. I think pets prbably make great guides since their love is absolute and without judgement.
I have dreams while sleeping where I see and interact with loved ones passed. They are not often, but when they happen I remember in detail. Usually there is a message with it though, or it calls my attention to another person that I need to contact... I think the interpretation is the hardest thing for me.. many times I should just trust my first inclination and not over-think it.
Nancy, no it dosen't always mean that someone in my family is about to pass when I dream of a family member now passed on. It depends on the dream and what I feel the message was when I wake up and think about the dream and its meaning. In your case, I believe that your mom was just stopping by and saying "hi". You've probably been thinking of her a lot lately and she was just letting you know that she's fine!