I am a vegitarian for 1 year, i now have hyperthyroidism. can my diet cause this?

Unlikely: Very unlikely. If you had a goiter, and if you lived in an iodine deficient part of the world (the us is not one of these), and if you greatly increased your dietary iodine, it could precipitate hyperthyroidism. More likely it is coincidental graves disease. Do thyroid problems run in your family?

Unlikely: While we know that autoimmunity (where the immune system makes antibodies that target a person's own proteins) causes graves' disease, the most common cause of hyperthyrodism, we do not know what triggers the autoimmunity. "stress" may contribute in ways not fully understood, and if your vegetarian diet was insufficient in some nutrients, that might contribute to developing graves otherwise, no.