Implants are essentially immovable, but that doesn't mean something on top of it couldn't have shifted. Sometimes the crowns on top of the implants cam shift-it would feel like the same thing.
Go to your crown dentist right away. Have him check the crown and as much as possible, check the abutment (the post that connects the implant to the crown). He may need an X-ray. Sometimes it is very difficult to tell if there is something shifting. He ought to be able to check the alignment of the crown and the bite to see if any shifting has occurred.
For now, unless you are having pain, swollen gums or some other obvious sign of infection, I wouldn't panic. But, don't delay getting it checked. IF something is shifting it needs to be caught before something breaks.