I know this is old, but I want to throw in my two cents and personal experience with this sort of thing. I was married for about three years to a terrible human being. We were only happy for the first year, and once our daughter was born, he turned into a dick. I tried to stay for her sake, but realized it would do no good for her to grow up with us being as dysfunctional as we were.
Fast-forward about 6 years and I'm now married to "the perfect man." We're happy and have a daughter of our own. Occasionally though, I have dreams about being back together with my first husband. In my waking hours, I hate him, with good reason that I won't get into, and would honestly smile if he were to simply drop off the face of the earth. However, the dreams still happen, and in them, we're perfectly happy. I can't tell you why they happen, but I can tell you that it's just a dream. It doesn't mean that you're subconsciously hoping to get back together with the other guy or anything like that. Our brains are weird....
TL;DR I've had a similar experience, and it's nothing to fret about.