A hard and rigid lump on the skull is most likely a bony swelling and commonly a bony spur or an outgrowth of the bone. An x ray
of the skull can be useful to evaluate it better. If at all anything suspicious on the x ray then a CT scan
can be done. If there is no pain then there is nothing to worry about. The pressure is only obvious because there is an outgrowth there. And since you seem worried about the lump currently you may tend to amplify the sensations.
The other possible causes of a hard lump could be a very hard sebaceous cyst which is a clogged sweat gland
or hair follicle which has been packed with a waxy like substance.
All these conditions are benign and can be left that way. If there are any changes in it like headaches
or growth then get it evaluated by a surgeon.
Hope this helps,
Dr. A. Rao. Kavoor