By checking your profile, I see that you are unmarried and I am guessing that you did not have sex between this time (just have to mention this; sorry if this offended you by my mentioning this...see I have to cover all the basic issues first). I am assuming that you are having a 30-day regular cycle for a considerable amount of time, and that's why you are concerned.
It is not uncommon for menstrual cycle to be variable in lenght. Stress and anxiety are most common cause if there is no other associated symptom. If you have symptoms, you may consult a gynecologist. However, it is more than likely, that you are quite normal.
Hope this helps.