Menstrual bleeding typically occurs about every 28 days, and lasts for about four days at a time.
Vaginal bleeding that occurs outside of the menstrual cycle has many potential causes, and should be evaluated by a doctor.
Here are possible reasons for vaginal bleeding between menstrual cycles, courtesy of the U.S. National Library of Medicine:
Cancer of the cervix, uterus or, rarely, fallopian tubes.
A side effect of certain procedures, including cervical or endometrial biopsy.
Stress or changing hormone levels.
Uterine fibroids or polyps, or vaginal dryness.
Pregnancy complications, including ectopic pregnancy or miscarriage.
Disease or injury to the vaginal opening.
Using an IUD, or stopping or starting the birth control pill.
An underactive thyroid.
Taking a blood thinner.