If you ever take a Psychology class in the future, you will read about a man named Sigmund Freud. Hence the baby video of you with both of your parents, happy. You were subconsciously reminiscing old, happier times and perhaps missing the simplicity of life in the past. Another researcher, J. Allan Hobson, proposed The Activation-Synthesis Model of Dreaming which documents that dreaming is essentially the brain synthesizing and integrating memory fragments, emotions, and sensations that are internally triggered. So, it's fair to say that at least some of our dreams mirror our real-life concerns, frustrations, anxieties, and desires; the majority of our dreams are meaningfully related to our current concerns, problems, and waking lives. So to put it simply, your dream is absolutely normal and very common.
When you dream about someone you know, you have a place to start interpreting the dream. They may have appeared because you like them or because they represent a certain kind of quality that you want to have in your waking life. When you do not know the person who appears in your dream, it can be extremely disconcerting. There are a few reasons why you may have an unknown person appear in your dreams though. Your subconscious creates the dreams that you have each night. Currently, neurologists believe that the neurons in your mind fire at random. The images, memories and thoughts that are released are jumbled and completely random.
If your ex is with someone else but still contacts you, then your ex likely wants the best of both worlds. He or she wants to explore the new relationship, as well as talk to you and pretend everything is alright at the same time. And since he or she can only be with one person at a time, your ex is most likely using you without his or her awareness. He or she is instead focusing on his or her new partner so much that contacting you became more of a time-filler for your ex. You are now someone your ex turns to for emotional support, advice, and boredom.