Suggest Treatment For Dizziness On Standing Up

Hi, im Haley. 20 years old, F. I have been expierincing something I cannot explain...I work m-f 9-5 infront of a computer. Now this is not happening everyday, but often when I stand up from my seat I loose my vison for about 5-8 seconds and feel extreamly dizzy. No headache comes afterwards but its obviously very scary and I really dont feel well the rest of the day. Still dizzy. Im concerned. I have been on a new diet. The south beach diet, but this was happening before I started the program. Can you help?