hello and thank you for sending the question.
There are several possibilities related to what you are describing. If this bump is right above a bone could be calcium deposit. Another possibility would be sebaceous cyst
. These are very common and often occur right below a hair follicle.
Even a cyst can be very hard in consistency. I would have your doctor take a look and if this is something that is uncomfortable your doctor might recommend removal. If it is otherwise not uncomfortable and your doctor agrees that it is not anything worrisome you might just continue to watch things for now.
I have a low suspicion that this is something XXXXXXX but it must be evaluated by your primary care
Thanks again for sharing your healthcare concern. I am available if you have any additional follow-up questions.