Why do I feel like passing out every time I stand up?

Every time I stand up I feel like I'm going to pass out and everything starts going black. I'm scared Im going to die every time it happens. I don't exercise anymore because of this. I stay at home all day sitting down or laying on the bed because I don't want to get up. One time I stood up and completely blacked out and my body started convulsing and I couldn't stop it. I wasn't unconscious and I was standing up the whole time but it scared the he'll out of me? Can any one please tell me what is wrong with me?