Although it is sometimes normal for teenage women to have irregular periods when they first start menstruating, your case is slightly different because it sounds like you initially had relatively normal periods but then they became less normal. This definitely merits a checkup and discussion with your adolescent doctor
or your OB GYN doctor.
There are several different causes of irregular periods (other than pregnancy, which sounds unlikely in your case). These could include cysts in the ovaries (which can sometimes be more common in teenagers), or hormonal imbalances related either to your thyroid gland or to something called polycystic ovarian syndrome. In addition to a physical examination, your doctor will help decide if additional testing is needed at this time. This could include basic blood work, for example. It could also include an ultrasound
to look at the anatomy of the uterus and ovaries.
These investigations will help your doctor figure out if there is a medical reason underlying your irregular periods and, if so, if this is a problem that needs to be treated with medication.
Make an appointment with your doctor at your earliest convenience. I'm sure they will be able to help you get some answers.