My Legs Always Wobble When I'm In Shock?

Well, actually I'm not sure if its shock.. But whenever I do solos in my chorus cl and I have to go up to the microphone my legs start jerking or something. It's really bad too.. I feel like I need to sit down. I've almost fallen over in front of many people a few times. But it's not just when I stand out.. Today I was at the mall in line at the food court and some man ped out right beside me. I was in so much shock that my legs started wobbling and the peremeditics had to ask if I was okay. Why is this ? And can I make it stop ? Btw I'm 14 if it makes a difference.