Often Times When I Stand I Will Become Extremely Dizzy

Often times when I stand I will become extremely dizzy to the point of almost fainting or even actually fainting and my entire body shakes almost seziur like I've been to the doctor about this several times and I've always been told it's cause I stand up to fast but standing up slow doesn't prevent it from happening and I'm not dyhydrated I drink close to a gallon of water a day also this might be irrelevant bUT I bruise extremely easy badly but I had the doctors check for anemia and was told that's not my problem so what else can this be