Sometimes, when I stand up and start walking after I've been sitting or laying down for a while, I collapse. I have been told it is normal for this to happen sometimes, due to blood rushing to your head (or something like that), but it seems more severe for me than for other people - after I take a few steps my vision starts to fade to black until I can't see anything, my legs start to shake and I collapse onto the floor, even if I'm holding onto something. I'm a 20 year old female so my bones are strong and can handle it, but I've hurt my head a few times because of hitting the ground so hard. My body sometimes convulses and twitches for a few seconds after I've fallen. After about 30 seconds my vision comes back and when I stand up again I'm fine. This happens fairly frequently (a few times a week), but it varies in severity (sometimes it's just my vision and I don't fall down).
Is this normal as other people say, or could it be a sign of something more serious? I seem to be healthy other than these episodes.