Why do i get so nervous in the exam?

i am a very hard worker,i study very hard,i practice n practice the things,even then during the exam i get nervous,i cannot even recall the important formulas which i would have used many times,revised many times,when this happens i feel very nervous,my heartbeat suddenly becomes fast,i get some sort of vomiting feeling,i am not able to know why the things i used to practice almost daily ,are not even remembered my me during the exam..n after the exam i come to know that this formula would have been used there n this thing i remember by myself only after the exam...sorry i am an indian,if there are any grammatical errors.bt please help me...my result is not good because of this..