Hi,
It may be a hematoma or a bony swelling. It is better to get the swelling examined by a neurosurgeon. Sometimes there is accumulation of fluid after surgery which is called edema. It may take a long time to resorb or may need to be drained; or it may need a course of antibiotics. A diagnosis and treatment plan can be arrived at only after examination of the swelling. Please take professional advise. Hope this helps you. Take care and regards!
I had nearly half of my skull removed and replaced after 7 months with an acrylic plate. It will take maybe up to a year for the swelling to fully go down. It may be sensitve for a long time also... Hope this helps!