It happens because of fear of losing. When you are able to focus and study for 10-14 hours, then you are the winner.
Don't be panic. Think positive. Feel confident that you will be able to clear the exams easily, You have prepared well.
Do breathing exercise with proper guidance.
Do walking or dancing or some exercises.
Take breaks in between every two hours of study for 1 hour.
Drink plenty of water.
Take good nutritious food.
Do meditation.
Next Steps
Practice these lifestyle changes. If you are not comfortable then consult a psychologist for counselling.