What Causes Sudden Dizziness?

Hi. I've been getting dizzy spells lately and am not sure why. It doesn't happen when I stand up right away but the other day I was at a cold restaurant, walked up and outside, walked toward my car, and then I felt dizzy. It was really humid outside so I thought maybe the weather was a factor. I also suffer from allergies so I thought maybe this was the reason as well. A few minutes ago, I was just sitting here at my desk and I turned my head slightly to the left to look at something and I felt a dizzy spell coming on. I've set up lab work today just to be on the safe side but I was wondering what it could be. Thanks.