Pain After Placing Nu-Oss Bone Graft: Source?

Authors

Dr. P asks:
I extracted retained root tips #18 (see x-rays below) and placed Nu-oss bone graft with membrane one week ago. I could not get primary closure of soft tissue. Patient was placed on antibiotics. The patient just called stating he is in severe pain? What could be the source of the pain? And any input on handling this kind of situation is appreciated.

Extraction #18/Bone Graft