Three months after root canal the tooth is moving, what does this mean?

A couple of months ago permanent crown was placed on my root canal treated tooth and things were fine for a little while. Then over a month ago i had my bite readjusted due to severe pain. Now these past two weeks the same tooth is moving slightly whenever I eat. There is no pain, but I am worried about the tooth coming out. I had a different dentist take a look while they were treating another issue. He said I need to go back and have another root canal. What do you think, could that be it?