Can a dental bone graft get rejected or infected, if put on top an extraction, pain after 11 days, on clindamycin day 5 but not going away.

Yes: Consult with your oral surgeon. There is no guarantee a bone graft will take even if no infection is present. I would be concerned as to why you have pain 11 days later and feel the mouth should get an exam.

Yes, it can: See your surgeon for an evaluation. Although not common, these things do occur.

See DDS asap!: You shouldn't be having any substantial pain so long after an extraction. Has your dentist seen you 5-10 days after the procedure for a post op visit? This needs to be evaluated asap.

Yes: They can get infected, but after this long, it may be a problem with the remaining jaw bone - either a fracture in the bone, a loose piece of bone floating beneath the gum, or even a piece of a root remaining. Clindamycin is very good at controlling bone pain and i'm concerned why you still have discomfort. Definitely see your doctor who removed the tooth for advice/examination.

Probable infection!: As others have said, after 11 days + some antibiotics, there is probably an infection. Oftentimes simply cleaning out the graft site followed by a few weeks of natural healing(and maybe more antibiotics, ) will make the site more receptive to a new graft. My experience: sockets grafted immediately after an extraction heal quicker and far more comfortably then "raw" non-grafted sockets.