I shake when i get stage fright?

Im on the mock trial team as a witness and i have to have composure on the stand but when i get up there i start to shake. my whole body just shakes, my teeth start chattering and its not even cold. i tried to nonchalantly hide my shaking by shaking my leg or kicking my foot acting like its just a quirk about me but my coach always catches me and tells me to stop. i don't know what else to do because i tried taking deep breathes but i get so nervous i forget to breathe! I'm a mess! I need help!