Hello, My answer is as follows: It is highly probable that you have experienced the so-called inter-menstrual bleeding. This conclusion is based on the information that your cycle has been always about 28 days, your last real period started at 25th of November, and therefore your next period should have started on 23rd of December. So, a heavy bleeding from December 6 to 9 (lasting 4 days) could only be an inter-menstrual bleeding. Inter-menstrual bleeding has numerous causes, such as stress, hormonal imbalance, change in medication, miscarriage (pregnancy), cancer, a growth in the uterus, infection, and etcetera. Since you did not provide any information about medication or pregnancy, then I would exclude these two causes. Therefore, you inter-menstrual bleeding could be anything involving stress, infection, hormonal imbalance, cancer or various entities in the uterus or cervix. In order to find out, you absolutely need to visit your gynecologist. He will have a history of your cycle and will make some necessary examinations (pelvic exam for example) and other tests (ultrasound, hormone level, thyroid function, tissues taking for biopsy, etc.) in order to determine the cause of your bleeding. Hope I have answered your query. Let me know if I can assist you further. Regards, Dr. Ervin Toçi