Spotting has a number of medical causes, ranging from harmless to potentially serious:

Implantation bleeding: Many women experience spotting in the early weeks (first trimester) of pregnancy. Implantation bleeding is a common cause of spotting. This occurs early in pregnancy, when a fertilized egg attaches to the wall of the uterus (known as implantation). A woman may mistake this light implantation bleeding for a menstrual period and therefore not realize she is pregnant.
Birth control: Certain birth control methods can cause spotting, including birth control pills, especially when they are first used. Switching birth control pills or using continuous birth control pills can also cause spotting. Some women with an IUD (intrauterine device) for birth control also report spotting.
Polycystic ovary syndrome (PCOS) is a common condition in which multiple cysts form in the ovaries. It can make it difficult to get pregnant and can cause irregular periods and bleeding.
Benign growths in the uterus or cervix, like fibroids or cervical polyps, can cause irregular bleeding and spotting.
Infections, including sexually transmitted diseases (STDs), are another possible cause of spotting or bleeding in between periods.
The menopausal transition is a time when women’s hormone levels can vary widely, sometimes causing spotting or irregular periods.
Trauma of any kind, including assault or rough sex, can injure the vagina and lead to bleeding or spotting.
While the vast majority of spotting in pregnant women is not due to serious causes, spotting and bleeding in early pregnancy is sometimes related to miscarriage or ectopic pregnancy (a pregnancy that has implanted outside the womb or uterus, most commonly in the Fallopian tube).

If you have continuous spotting or vaginal bleeding, or spotting postmenopausal you need to be evaluated by doctor or other health-care professional.