At first i thought i had hypothalamic amenorrhoea which is when your body is under stress either mentally or physically and so the hypothalamus stops sending signals to your glands to produce hormones. It focuses on other more vital organs. This could be due to being under weight, or being a healthy weight but having alot of stressor, or sudden changes in your body such as weight loss ( even if you're losing weight due to obesity, that's still stress on your body because it think omg I'm losing fat and so it stops all unnecessary system such as the reproductive system)
Stressors include exercising, that's why when women start exercising they have to start slowly and build up gradually. Also eat more, eat carbs before your workout and carbs and protein afterwards. That's if you're not obese and trying to lose weight.
I found out if you're a BMI of less than 22, you're more at risk. Female bodies aren't supposed to be 'lean' we literally NEED fat for hormones. The brain can then say okay now that i have enough fat, i can bear another human being lol and then you start having a fertile cycle again. Ofc some women can be lean and have a healthy cycle, if they have a good diet.