I can give you some thoughts as to what might be going on, but if it has been present for a few days, I would definitely recommend making an appointment with your primary care physician
to get it checked out. Your physician will be able to give you a much better explanation as to what is going on than I can since they will be able to take a history, and examine your head thoroughly.
Thinking very generally about the posterior aspect of your scalp, there are a number of different things that can potentially give you pain and sensitivity in a localized area. One of the more common, and potentially worrisome (if not treated early) problems is something called cellulitis. This happens when certain types of bacteria that can be normally found on your skin are able to enter the actual tissue of your skin and cause a tissue infection. This is treated with anti-biotics, but can potentially spread fairly quickly and become a significant problem if not managed appropriately.
You could also have something more superficial like a folliculitis, or a dermatitis which may be managed with a cream. Of course there are muscles that attach to your skull, and one of them is strained, it could also cause a sore spot which is not particularly worrisome. As you can see there are many (and this is just a few) potential causes for your issue, and they range from serious to not-serious. I would definitely recommend having a physician take a look at it if it doesn't get better. Your primary care physician is a good place to start, then they may refer you to a specialist if needed. I hope this helps.