Can permanent crowns rock back and forth?

I had four permanent crowns placed on my top front teeth. I don't think they are loose, but sometimes they feel as if they are shifting or rocking back and forth ever so slightly, from the pressure of my tongue (in one direction) and the pressure from my top lip (in the other direction). I've read that it's normal for teeth to move or shift a bit. Is this true? It just feels weird.