Hi,
This is something you should see your primary doctor or gynecologist for; the term for this is 'secondary amenorrhea' (meaning missing 3 periods in a row). There's a long list of conditions that can cause amenorrhea, such as: medications, hormonal imbalance, extreme weight loss, an underlying health issue, etc... During your appointment with your doctor they may ask you for info. about your last period, and your health history~ this is so they can determine what, or if any, tests are needed.
Take Care