sometimes, when i get up, after sitting down and relaxing for a long period, i get dizzy, everything goes black, and i can't hear anything. I have to hold on to something to make sure i don't fall. This is the light version, sometimes i start to convulse(i think it's a convulsion, might be just tremors), and have to sit down on the floor, to make sure i don't fall. I never lose conciousness, i am aware of my surroundings, but i can't controll the tremors usually strong when they happen. Could someone give me an idea why this happens? It happens at least once a year, mostly summer-time but today was the worst one.