A number of things can cause it to happen. These are the most common:
- Hormone-based birth control
- Some sexually transmitted infections (STIs), like chlamydia
- Infection of the cervix or lining of the uterus.
- Blood clotting disorders, like von Willebrand disease
- Other health conditions, like hypothyroidism, liver disease, or chronic kidney disease.
- Fibroids or polyps
- Polycystic ovary syndrome (PCOS)
- Cancers of the reproductive system
- Perimenopause