I have a lump in the back of my head

Patient: I have a marble sized bump in the back of my head, and it’s really hard (feels like bone) and it doesn’t come with any pain, or headaches or anything but it is rather annoying to have. I just discovered it not to long ago like a month ago and I’m not sure what it is. I want to get a haircut too but I’m not sure if it will be noticeable or not. Can someone please assist me with help, thank you

Symptoms: Just a bump