Abnormal vaginal bleeding, such as having a period early, has many possible causes, including bleeding associated with an undetected pregnancy and bleeding caused by an intrauterine device. Bleeding between periods may also indicate a hormonal problem, such as polycystic ovary syndrome, according to WebMD.
Ovulation frequently causes mid-cycle bleeding, which is sometimes mistaken for an early period, explains WebMD. Breakthrough bleeding, or bleeding between periods, is also common in women who take birth control pills, especially if they use the pills to delay or temporarily stop their periods, states Mayo Clinic.
Other causes of irregular bleeding include uterine fibroids and pelvic inflammatory disease, or PID. Uterine fibroids are noncancerous growths in the uterus, while PID is a serious infection of the pelvic organs that often goes undetected for many years, says Mayo Clinic. Other signs of PID include fever, abdominal pain, foul-smelling vaginal discharge, pain during intercourse and painful urination, according to the Centers for Disease Control and Prevention.
Endometriosis, a condition that results in the growth of uterine tissue in the abdominal cavity or on pelvic organs other than the uterus, is yet another possible cause of bleeding between periods. Endometrial cancer and the precancerous condition endometrial hyperplasia also cause irregular periods in some women, notes MedicineNet.