How Do I Know if my Chin Implant Moved After a Really Hard Blow? (photo)

I would say it would be possible, but unlikely, that the chin implant you had placed 2 years ago moved after you fell.  You would need to be examined to know for sure, but you could just wait another week or so to see if the bump goes down by itself.  I would say it's more likely that you had a small hematoma after the fall and it is just slowly resolving.  

Your issue need in person evaluation to determine if there was any injury to the chin implant. Seek care. 

If your chin implant became displaced, it would be more likely to create a visible asymmetry or irregularity of the shape of your chin or jawline, you might be able to feel the edge of it inside your mouth by your gums, or you might be able to actually feel it move or shift.  If all that you feel is a hard lump in a fixed position, I would bet that it represents a small hematoma, or collection of blood, maybe around the implant or capsule, and it is in the process of resolving.  This could take a few months or so , and you should be patient, watch for changes, and have close follow up with your surgeon.