It is perfectly normal to have irregular periods at your age. It only becomes a concern if it does not become regular by age 21. If this is your only concern, then no worries. Only worry if anything ELSE is abnormal: like a foul odor, itching, sores, burning sensations, changes in your normal color or amount of discharge. That's time to go see doc. Blessings - Blu
You seem to be having secondary amenorrhea. Secondary amenorrhea is when a woman who has had normal menstrual cycles stops getting her monthly period for 3 or more months.
Causes of secondary amenorrhea include stress and anxiety, malnutrition, depression, overexercising, sudden weight loss or gain (obesity),thyroid disorders and hormonal imbalance due to polycystic ovarian syndrome(PCOS).Smoking is also associated with secondary amenorrhea. At your age, PCOS is an important possibility along with hormonal imbalance. I suggest you to get thyroid conditions and PCOS ruled out from a gynecologist.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.