I think your mind is just going through different possibilities: what if you were to go back with your ex. I think your grandparents are just there in the dream to present an alternate possibility, to suggest that you are better off with your ex. It's normal to dream about different possible outcomes of events in our lives. It doesn't mean that you would go back with your ex in real life or that you would even consider it. Just the fact that you felt so strongly about the matter in your dream shows that you've made up your mind in real life.