Social Fear

I am currently taking two classes at USU right now. Just two lame generals, PSY 1010 and a Planet Earth class, easy enough right? Typically I go to class, sit some where in the back, and don't open my mouth at all. I don't consider my self to be a genius by any means, but I'm not stupid either, I'm shoved somewhere in the middle, which I am A. O.K with. So, I decided that I learn better when I'm an active participant in lecture, imagine that, so I started to speak up and for the most part its alright, but the other day in psychology I said something that was contradictory to what the teacher was saying... and the entire class turned back, looked at me while I'm trying to explain this whole nature vs. nurture and how it leaves to many extraneous variables, and as soon as they all looked back I got totally frazzeled and mixed up my words, then of course I blushed , my heart started racing and just stopped talking because at this point I had completely lost my train of thought. I felt really stupid and was really irritated. I'm not a shy person, and typically I'm not embarressed very easily. I usually don't have a problem speaking in front of people but everyonce in a while this happens to me and I hate it. I can't figure out why it happens, but I need to fix it.