There can be several reasons. Of course, the most obvious is bumping your head, which you can actually do in your sleep and not realize you did. There is also the possibility of an ingrown hair, which will cause a bump and eventually become cyst like and will break on it's own. Best to have checked by a doctor, or walk in clinic. I get them some times and when they break, I apply rubbing alcohol and keep area clean and dry. When I was a teen, had a real big one on my head, and it broke. My mom made me go to school in -20C despite the fact that the doctor told me to stay home and apply warm compress. You may want to try this. A hot moist cloth to the area, to bring swelling to the surface so that it does not continue to be a problem. If it should break, wash hair without shampoo, warm water, and wrap head in towel. And try to keep it aired out and warm, stay indoors, until it heals
