What's wrong with me?

I am not a very shy person. I have a lot of friends who I am usually the loudest amongst, I talk to the girls at my school with confidence, and I can deal with being in crowds at movies, in the mall, etc. But for some reason, when I am forced into 'pep rallies', sitting in a large m of people in bleachers, surrounded on all sides and looking across the gym at another mive block of people, I get these mive 'waves of fear' - best way I can put it. They seem like sudden bursts of nausea, even though I don't actually feel it in my stomach. A gripping type of fear. I also get it when in the auditorium and surrounded by people, and sometimes in specific cles (only two, and not nearly as severely.) I've had a history of poor public speaking, reaching back to when I opted out of an eighth grade play lead-role and actually contracted shingles, presumably from stress over it. I try to avoid speaking to crowds. However, I don't show the regular symptoms of social anxiety. I don't have an issue in most public places, I don't mind being the center of attention among my peers and I have a good amount of confidence. I'm terrified about graduation coming up, surrounded by hundreds of screaming people and stuck in the center-floor and forced to walk in plain view to get my diploma. I don't know how college will be. What can I do, and what is causing this? I don't want to turn to medication for one day of my life but I fear it might be the only way I won't seize up, throw up or something among those lines.