Laura you may think things aren't triggering them, but stress in general life can be as big enough trigger as any. You may need to see someone about it (it's nothing to worry out!) and they may give you coping mechanisms
If you don't want to, do your parents know?
Anyway, if you feel yourself having one try taking deep, calm breaths and keep trying to remind yourself where you are etc. Trying to keep in touch with reality can postpone a panic attack - but the best thing to do is let someone know.