Before I answer your question, let me first say that head injuries can be very serious, and should be respected as such. For this reason if you have "the worst headache
of your life", or develop any mental status change, speech problems, or vision change, I would recommend going to the emergency department immediately. I say this mainly because without being able to talk to, or examine you, there is no way for me to know how hard you hit your head. The posterior skull does have soft tissue and muscle over it which can swell as a result of trauma. Since the skull is a closed structure, any swelling outside of the bony skull should not have any effect on the brain (which is a good thing!). When little kids hit their heads, commonly this soft tissue swelling is referred to as a "goose egg". In some situations, the swelling can be quite profound, however with ice, and over the counter anti-inflammatories (like ibuprofen), it typically resolves within a few days to a week without any complications. If the swelling seems to be getting worse, or even rapidly expanding, I would again recommend going to the emergency department. Otherwise if it is taking a long time to go down, and you just want someone to take a look at it, your primary care doctor
is a good place to start. I hope this helps.