Does anyone know why a menstrual cycle changes in a young girl?

Very common. Menstrual cycles are typically unpredictable in adolescence. Just be patient or talk with your pediatrician or gynecologist if worrisome.
Pacing. In the first months after they start, teens often don't ovulate (release eggs) which is part of the hormonal system that paces regularity. Without ovulation the cycles are driven by estrogen levels and may be erratic. After 6 mo to a year, most are ovulating & set a pattern. However, some go on to remain somewhat irregular through life.