I did have nightmares about my ex, long after I thought I had moved on from him. I don't know why, it just happened. We had cut off contact with each other, and I had even been doing well at catching myself when I'd start to think of him.
To me it meant, that I hadn't fully moved on, and that I still had more to do. But dreams are different for different people. What does your ex mean to you? Does he represent betrayal? Grief? Sadness? Maybe his appearance in your dream is just your way of recognizing that you've been really hurt by him. You already know that, but maybe it's a reminder to keep processing those feelings. It doesn't mean you're back at square one, it just means that he meant a lot to you, and it hurt when you lost him, and it still hurts. Hopefully not like it did before. Maybe this dream was a way of showing you that you've already come a long way to get over him?

It's human instinct that we often dream/nightmare about those who held a great role in our lives. Wether it be good or bad. It's been said that when someone you know is awake in your dream, you're awake in theirs. Maybe he's having the same nightmare. Maybe he's feeling guilty. Or maybe you're just having trouble letting go. Maybe you should spend a day thinking of all the good times. I guarantee they out-weigh the bad time. Maybe you're nightmares will become dreams again. Keep your head up, don't let it get the best of you. You're better than that. Many blessings and prayers sent up for you. (: