Dizziness and vision blackout when standing?

I'm 18, and this has been happening for awhile, I'll sit down for a couple minutes and when I stand I get all dizzy and my vision goes black for a little, sometimes I'll have to catch myself on something so I don't fall, or I have to sit back down. Sometimes It makes me feel sick too. it doesn't happen too much, but I first noticed a couple years ago. And it doesn't always happen when standing, it can happen randomly if I'm just walking through the house, is this normal? I drink water everyday, I don't think it's dehydration like my mom keeps saying... But I'm not sure. Thanks!!