Hi thanks for your question
You have bony swelling on the back of your skull,near your occipital protuberence.
It is symptomless.
Most probably,this bony bump is bony exostosis
from occipital bone
and there is nothing to worry.
confirmation of diagnosis of exostosis can be done by X-Ray Skull.
If X-ray doesn't confirm the diagnosis,CT scan
skull will confirm the diagnosis
Hope this answer your question