I wonder if you are feeling one of the plates they use to fuse the bone flap onto the skull. They often use discs. After a few weeks, and the swelling over my husband's incision went down a bit, we could see the line of demarcation around a couple of the plates, and in one place, you clearly feel a screw on his head (yes, it is weird.)
Is the ridge you feel mobile? You're saying it moves around under your skin? That would definitely be disturbing. Is there redness, tenderness, or heat?
My husband complained a lot about discomfort after the surgery, it wasn't really painful, just annoying and uncomfortable, itchy and a little piercing, while everything sort of knit itself back together. The scalp has a lot of little blood vessels.
So, all in all, it might be more healing from surgery related than radiation. I hope it resolves for you soon.
OMH