premature greying can occur due to a lot of reasons which include genetic predisposition, certain illnesses like vitiligo etc. and are not just limited to stress and dietary deficiency. so would like to suggest that you take care of ur eating and sleeping habits. don't take so much of stress because that would increase ur problem further and you would be under more stress cuz of gyreying hair.
I suggest you see a dermatologist for further evaluation. if everything seems normal then I suggest you stop worrying about it. if someone asks you just smile and say its in trend.
all the best.