Could someone tell me why I started shaking? Any psychologist or psychiatrists to help me? Look @ details?

I'm a business major so obviously there are many presentations and group projects involved. I was always pretty confident in my presentations. I've gotten nervous but it always went away quickly once I started. I even spoke to large groups of people at a school emblies before. I even gave a presentation on bankruptcy laws and regulations which I knew nothing about and I still got the highest grade. I am taking currently taking a cl that is considered to be the hardest cl in our business adminstration department. I had the easiest speech ever...it was a self introduction. I started out articulate and confident and 30 seconds into it I started noticing my neck tensing up and my head started to shake like I was laying on those soft maging chairs. I had to take a deep breath & I blanked out for a good 3 seconds then I was a better throughout the rest of my presentation but it still wasn't as smooth as the rest of my speeches. What could have triggered my tremor out of nowhere?