Is it normal that i can't help thinking about my ex?

It is now almost a year ago that my first boyfriend, of three years, and I decided that we were not the best choice for each other and we should find someone to make us happier. We broke up with mutual understanding.
About three months later, my best, oldest friend, who I've known since birth and who lives far away, and I, decided to have a reunion. It was more than great and we fell in love and soon were together. We have been absolutely happy and we are a much better match than my ex and I were (a relationship based on a lifelong friendship versus one based on one date). However, a week ago I moved house, and for some reason, since then I keep having to think about my ex again. I cannot get him out of my head - and of course the more I try, the less it works, so that he even pops up in my head the moment I wake up and sometimes even when I am masturbating. I am not actively thinking of him, nor do I long for him or want him back. But it just doesn't seem fair towards my boyfriend that I keep having to think about my ex. (I have not seen my boyfriend this week while this was going on - which is normal for us because of the long distance. So that can't be it.)
Am I unique and weird in suddenly having to think of my ex again? Or does that just sometimes happen and will it go away by itself again? I can't help but worry about it. Which of course makes it worse. Help!