That happens to me with words or clips of songs sometimes. If I do something else, it stops. I don't have trouble concentrating when it happens, if I can find something productive to do. I think it might be a form of intrusive thought. I read a study that said everyone has intrusive thoughts at some point or another. Unfortunately, I have no idea where I read it, so I can't post it here. 
In short, don't worry about it. It's common.
In short, don't worry about it. It's common.
