What Causes Pain At Back Of Head?

I am having pain at the back of my head. It may be a muscle pull or spasm. I use to go to gym regularly. While doing some heavy exercise sometimes I used to get a sudden pain at the back of my head like a jolt of blood ran into it. It used to cause slight headache as well from which I relieved myself by taking some painkillers. But last week it happened again, but this time it got severe and since then I have slight pain at the back of my head. I feel it whenever I move my head up or down. It causes headache as well and does not let me concentrate on my system or laptop.