Wtf are you expecting? Since when was the logical next step for a medical diagnosis after Google to go ask a bunch of kids on an internet forum? Go to a goddamn doctor and stop being a troll.
You never know who could have medical experience or have had the same problem before.
OP, you could have pinched a nerve. Were you walking the dog after sitting down in an awkward position with your neck bent or anything?
Edit:
It doesn't have blood in it. It stops after one minute and it happens when I exercise
Yeah dude, you've pulled a muscle that's causing pain in the back of your head. I'd do some stretches every now and then and rest. It's best not to stress your body to try and "fight" through the pain. That won't do you any good. You're fine, just get some rest. If it isn't gone by the day after tomorrow, if not sooner, I'd recommend a doctor at that point.
