If pregnancy test was negative the probability that you are pregnant is very low. However blood analysis for Human chorionic gonadotropin
(to exclude pregnancy for 100%) and ultrasound examination should be done. Ultrasound can reveal possible reasons of your irregular periods
and amenorea because it can be caused by polycystic ovary syndrome
, thyroid malfunction, pituitary tumor so you may need more detailed examination with hormonal panel and even computed tomography
. Sometimes such problems also can be caused by stress, poor diet or excessive exercises.
Wish you Good Health
Dr O. Aleksandrov