What Causes Pain In The Site Of Bone Grafting?

I fractured my foot and ankle slamming directly into a piece of wood in September of 2010. The fifth metatarsal fractured at the distal head and was quite significant. I was treated with a boot, and told I did not need surgery. The metarsal was slow to heal but the ankle healed and the other fractures. Last x-ray in November and December showed a healed fracture. I began having pain and swelling in April and May of 2011. X-rays revealed a bone mass in the fracture site which had characteristics of a tumor. Emergent surgery was performed, a bone graft completed (taken from heel), and biopsy and cultures were clean. I am 13 weeks status-post surgery, and recently began to have significant pain in the graft site. Is this normal? If the site is a non-union, what is the next step?