You may get a positive pregnancy test if you are not actually pregnant. The tests usually detect the hormone human chorionic gonadotropin (hCG), which is produced in your body by the cells formed in the placenta during pregnancy. This test can sometimes be falsely positive for several reasons:  

1. After an abortion
2. After a miscarriage
3. 4-6 weeks after childbirth
4. Medical conditions can cause a higher hCG:
   • Ovarian cysts
   • Cancers  
   • Pituitary gland disorders (in perimenopausal or menopausal women)
   • Hormone levels
   • Gestational trophoblastic diseases 
   • Phantom hCG 
   • Some urinary tract infections
   • Kidney disease
5. A false positive test can also appear due to a pregnancy complication when an egg gets fertilized but is unable to implant. It may happen as a result of some issues concerning the uterus: 
   • Uterine fibroids
   • Scar tissue
   • An irregularly shaped womb  
   • Other causes of this type of pregnancy are unknown
6. Certain medications that influence hCG levels.

Sources:

Five reasons for false-positive pregnancy tests, medicalnewstoday.com

Human Chorionic Gonadotropin (HCG): The Pregnancy Hormone, americanpregnancy.org

Medications That Will Affect HGC Testing for Pregnancy, livestrong.com