Most of the time, irregular periods are part of the normal changes that can happen when you're a teen. As you get older, your cycle will probably settle into a recognizable pattern.
Sometimes, irregular periods can be caused by some medicines, exercise too much, having a very low or high body weight, or not eating enough calories.
Hormone imbalances can also cause irregular periods. For example, thyroid hormone levels that are too low or too high can cause problems with periods. Some girls have extra androgen, a hormone that can cause hair growth on the face, chin, chest, and abdomen. Extra androgen can also makes girls gain weight and have irregular periods.