I assume that you had a fixation system associated with the ACDF fusion (either a plate or a graft with self retaining screws into the vertebral body). Generally, flexion (bending forward) is safe as this puts the graft into compression which is a stable position. Nonetheless, development of pain with that position could mean you “slightly” pulled on the fixation and disturbed the surgical site. This is why I have patients wear the cervical collar for the first week to “remind them” that they just underwent surgery.
It would be a good move to go back to your surgeon and get a new X-ray to make sure the surgery is stable. It would be highly unlikely that there is any noticeable difference but helpful to know that there is no change in the architecture of the surgical site.
Dr. Corenman