What Causes Dizziness And Headache?

Hi, so I'm only 13, and a lot of times when I stand up I get really dizzy. It doesn't happen all the time but when it does my vison gets all blacke. Sometimes there is a little tunnel of vison. I know that it's supposed to help if I stand up slowly, and even though I do his I still get dizzy. After this happens I have a huge headache for about 10 minutes afterwards. This has been going on for several months. What does all this mean?