Your symptoms are exactly as you suspected. Extension of the neck (bending backwards) or neck retraction (“if I pull my head back like I’m trying to make a double-chin”) will narrow the foramen (the exit hole of the nerve). If there still is some narrowing from the surgical posterior foraminotomy, the nerve will still be affected by compression.
This is the dilemma of the posterior foraminotomy surgery. If the nerve is compressed by an anterior spur off the uncovertebral joint which is typically the case (see anatomy and cervical radiculopathy on the website) the posterior surgery can only decompress so much. Too much removal of the facet in the back of the neck will cause instability and subsequent neck pain. Too little removal can leave residual narrowing of the foramen.
You have to give this surgery some time to determine if your symptoms will abate. I would say that three months would be enough time. If you continue to have these symptoms, you might have to consider an ACDF.