Nonsexually active 17 year old with a 2 week early period that's incredibly heavy. Periods always regular until this. Feel nauseous and shaky. Why?

Various reasons... Changes in your cycle can be related to hormones, stress, diet changes, and medical conditions. With the heavy period and effects you are describing I would advise that you see your physician today - have your hemoglobin checked to make sure that it isn't too low and then take the evaluation from there. Take care!