I think the most important thing is for you to be evaluated by a doctor
in person. If you are currently, or have been previously, sexually active (meaning you have had penis in vagina sex), you should take a pregnancy test. This is because a missed period can often be a sign of pregnancy, and it is important to know early on in pregnancy if you're pregnant. You want to know if you are pregnant as soon as possible so that you have time to weigh all your options and make sure to have a healthy pregnancy should you choose to keep the baby.
Another possibility, especially if you aren't having sex, or if you use very reliable birth control
, is that this is normal variation in your periods. Often young women have irregular periods for several years after starting their periods. If I saw you in my office, I would want to know at what age your periods started and if they have been regular before in the past.
There are a lot of other medical conditions that affect your hormones that can cause you to have irregular periods or miss a period. One example, is losing a lot of weight can make you miss a period. I think the most important thing is for you to be evaluated by a doctor in person and to have a pregnancy test, so that we can be sure you aren't pregnant and get a better sense of why you may have had a delayed period.