What Causes Frequent Menstrual Cycle?

hello...This is shilpa 22yrs old.I am a healthy and active girl who follows good diet.I usually get periods on time and never faced problems like over bleeding and all.But last month my cycle is supposed to start on 25th nov,but it started early i.e,15th nov and continued till 27th nov.I felt okay and dint go to the doctor.And suddenly yesterday i.e, 7th december the menstrual cycle started again.Can you please let me know the problem.And also is there any need to go to a doctor.

Thanks,
Shilpa.