What a great question you have, and a very, very common one. Many things can cause irregular menses, including stress (like college/finals), strenuous exercise regimes, changes in environment (like living away from home), changes in weight, changes in hormones, and sometimes thyroid problems. You have currently missed your periods for two months. You eliminated a common cause by doing a pregnancy test. When the stress of school subsides over the semester break your menses may begin. If not, you may want to make an appointment with a health care provider, such as the Student Health & Counseling Center, for an assessment of irregular menses.