I've had this random throbbing headache for 3 days now?

I've had this random throbbing headache for 3 days now it started October 10th and now it's october 12th and it still hurts a lot. It hurts the worst when I'm standing. The pain is the back of my head. I'm 14 and I haven't done really anything at all, all week. I just watched videos on my iPad and layed down all weekend. When I get up there's a sharp pain then it goes away after like 5 seconds then I comes back and does the same thing over and over. When I'm laying down I don't feel it that much. I don't play any sports or anything so there hasn't been any physical contact with others.

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