Can someone help me with a reoccurring issue, I seem to be disoriented when standing after sitting for a while?

Whenever I sit for a while, I stand, (duh). When standing, I experience either a complete conscious blackout, or everything goes bright. I feel a slight throb in my head, then I experience a small ring in my head as well. I body seems to feel numb but I go back to normal about 5-10 seconds later. I am only 15 and I dont know what could be the problem. I am not diabetic, but people both on yahoo and in real life have suggested hypotension or health issues.