The past two months my menstrual cycle changed, from being about 30-32 days long to 20-21days long? is this bad?

Probably not: Most women have cycles that remain about the same, but the cycles can change over time. The normal cycle length varies from about 21 to about 36 days - it sounds like you are within that range so you probably do not need to worry.

Not necessarily: The length of menstrual cycles can change over time and is not necessarily a sign that there is anything wrong. A shorter cycle may result in being more uncertain about when ovulation occurs. Shorter menstrual cycles result in more frequent periods, and if the volume of blood loss during a period does not change, there is a greater risk of anemia. It is a good idea to monitor your hemoglobin.