Call your dentist and see if you can email that picture to them to show them what is going on.
Did a stitch come loose by any chance? Looks like the bone is poking up between the 2 stitches.
From an article I read: "The block of bone that was removed from the chin will be anchored in place with small titanium screws. A mixture of your bone marrow and some other bone-graft material may then be placed around the edges of bone block. Finally, the surgeon may place a membrane over the graft and close the incision."
Where did the bone come from? I can't seem to find anything about the bone being rejected with this type of procedure (NOT A DENTIST THOUGH), but if it's a sample from you, there's no way it could be rejected I would assume.