Well, you could talk to your teachers to see why they predicted you much higher grades. What were their predictions based on and why do they think you did significantly worse? You need this information so that you can decide whether you just had an unfortunate "blip" in your exams and whether your performance is seriously inconsistent-or were your teachers just wrong?
You say your results shocked you,implying that you aren't the best judge of your own abilities, so you need some serious feedback from disinterested sources, hopefully your teachers.