Early on when you are first getting your period, it can be normal for it to vary a lot in terms of when it comes. However, by the time you are 18, things should have settled into a normal rhythm. Therefore, you should definitely make an appointment to see your OB GYN doctor
or your primary care doctor
to discuss the situation.
The mostly likely scenario is that you have a condition called polycystic ovarian syndrome. In this condition follicles in the ovary (the parts of the ovary that release the egg each month) begin to mature but, because of some hormone imbalances, they do not reliably release the egg (called ovulation). If ovulation does not happen, then the menstrual cycle will not happen or will happen only very irregularly. Polycystic ovarian syndrome is associated with trouble getting pregnant and it is also sometimes associated with being overweight and a risk to develop conditions like diabetes and high blood pressure later in life. In addition to good diet and exercise, there are medications that can be used to regularize your cycle. Some of these are medications that are normally used to treat diabetes, while sometimes birth control
pills are also used.
Your doctor can help you figure out the best approach in your case.