When I concentrate for 5 to 10 min on anything like, most during study, in exams, even in Lectures After 5 to 10 min suddenly I feel at my back of the head a pain but nt exactly pain that is different from pain like when we set for a long time then suddenly feel my leg got something like jhun jhun similarly that happens to me when I concentrate for some time. Due to this headache sometime I had black out also. I had to loos my concentration and get relax to get normal. I don't know what to do, please help me say.