What can effect your period ? i haven't been on my period for 4 months i am 23 years old i already took a pregnancy test it came out negative

Many things: Our periods are like the canary in the coal mine. They are very sensitive and are easily disturbed. Stress, changes in diet, exercise, or weight, thyroid disease, insulin resistance ( pre-diabetes), and some medicines are a few of the things that can effect your periods. If you haven't had a period in 4 months you need to see your doctor to find out why and get treated to prevent future problems.