I saw that you said you are 5'9" and 88lbs. According to the CDC website that is a BMI is 13 which is very underweight. Being very underweight usually results in loss of period and irregular periods. It is a common cause for irregular periods.
According to Havard Health what you experienced is Secondary amenorrhea which "occurs when a woman has experienced menstrual periods, but stops menstruating for three or more consecutive months"
This can be caused by:
- Pregnancy (the most common cause)
- Breast feeding
- Menopause, the normal age-related end of menstruation
- Emotional or physical stress
- Rapid weight loss
- Frequent strenuous exercise
- Hormonal birth control methods, including birth control pills, the patch and long acting progesterone.
- Polycystic ovary syndrome. This condition is associated with a tendency to be overweight, excessive body and facial hair and hormonal irregularities.
- Premature ovarian failure (menopause before age 40)
- Hysterectomy (surgical removal of the uterus)
- Abnormal production of certain hormones, such as testosterone, thyroid and cortisone.
- Tumors of the pituitary gland
- Chemotherapy
Female athletes, especially young women, are more likely to have amenorrhea. Exercise itself does not cause amenorrhea. But it is more likely in women who exercise very intensely or who increase the intensity of their exercise rapidly.
Secondary amenorrhea occurs quite often in women who engage in activities associated with lower body weight, such as ballet and gymnastics."
Link: https://www.health.harvard.edu/a_to_z/amenorrhea-a-to-z
You should consult a doctor if you are concerned. From my personal experience, I think it might be due to your low weight. The only time I missed my period was when I was trying to lose weight and ended up eating too little calories.
The best way to really figure out what is happening is to go to the doctor.
