Why can't I stand for a long time?

I first noticed this problem this year because i'm in AFJROTC. Every other Wednesday we have a uniform inspection and it usually takes about 30 minutes. I can never get through inspection anymore because after about 10-15 minutes I start to get sick to my stomach. If I ignore it though my vision will start to go fuzzy (Like the black and white fuzzy snowy screen on a tv) . I'll get a loud ringing in my ears to the point where I can't hear people talking to me. I'll feel like i'm going to throw up and I don't know what happens after that because they usually let me sit down. They said i get really pale and cold, but I'll sweat. I don't lock my knees when I stand, I actually made it a point to try and keep my knees slightly bent but it doesn't make a difference. During inspection, you can't move, you can't look around, you can't do anything or else you get points deducted for military bearing. I think it's only during that class because I can't move and I have to stand still because i'm a dancer and i'm used to being on my feet for 14 hours during the summer and I was fine, but I was also constantly moving. Any ideas to what might be causing this problem?
Why can't I stand for a long time?
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