Thanks for writing your query to us.
As you had mentioned that you had taken pregnancy
test twice it is negative two times , so there is a possiblity that you may not be pregnant.
Normally under stress
periods can get delayed for few days & since you are under stress of examination it is quiet possible that menses have got delayed because of stress. If you dont get periods in a weeks times than kindly repeat urine pregnancy test again just to reconfirm.
Take care & wish you all the best for your exam.