It doesn’t take much to throw off the hormone levels of estrogen and progesterone in our bodies. Weight loss, weight gain, being athletic, stress, change of seasons, changes in circadian rhythm, etc., can impact our cycles.

In my teens I was very athletic, with little body fat. That stalled my periods for several months at a time. Later, when out of school and aggressive training wasn’t part of my daily regiment, my period regulated. But there would be months when it would fall out of sync. It took me years to realize that I’d have disruptions in my cycle every fall as the season’s changed. The circadian rhythm was disrupted. My daughter also has the same thing happen. It’s not uncommon. Talking with other friends over the years, many of them have similar stories.

Hormones are responsible for regulating our menstrual cycles so when there is something disrupting that normalcy you may possibly go a month or two without a period. At your age, the odds of having something serious as the cause would be pretty minimal. Being 20 and having irregular periods isn’t alarming but you’ve noticed a change.

So I’d keep an eye on this for a couple months and if you don’t go back to a normal rhythm, or develop headaches, unexplained mood swings, that type of thing, I’d suggest making an appointment with a gynecologist. You’re in your 20s so it’s time anyway for establishing a contact with a women’s health professional.

Do you feel you have mood swings or PMS even in months you skip your period?
https://www.verywellhealth.com/reasons-you-missed-your-period-2757503