Almost passed out in the shower?!

I just about passed out in the shower. First, my eyes started going black then they went back to normal. A few seconds later, I started feeling really lightheaded, so I sat down for about a minute. While I was sitting, my stomach started feeling queasy, so I deciced to rinse the conditioner out of my hair and get out. I was rinsing the conditioner out of my hair when I started feeling lightheaded again and my eyes started to go black. Next thing I know, I fell over the side of the bathtub. I don't think I completely passed out, but while I was falling and getting back up it felt like slow motion. I got out and sat down for about 10 minutes and drank some water. While I was walking I have to hold on to things sometimes and my balanceis off. I'm laying down in bed now and still feel dizzy and my heartbeat is a little fast. I don't remember drinking water yesterday....just milk. Could this be dehydration? Or something else?