Long period

Menstrual bleeding for more than 7 days is known as a prolonged period, and could be caused by many things, including an underlying medical condition, infection, hormone imbalance, or unknown reasons that will remedy themselves (stress, medications, etc).

Do you have any other symptoms? It's great that you are not experiencing heavy bleeding, in addition to the prolonged period.

Did you have unprotected sex? Are you trying to become pregnant? Bleeding is not an indicator of pregnancy; do you have reason to believe that you may have miscarried? Some women do experience abnormal bleeding if they have a miscarriage, but I'm wondering how long ago you had unprotected sex (in other words, if you were pregnant, how many weeks would you have been pregnant?). If you just recently had sex, say in the last week or two, it is not likely that your prolonged light bleeding would be a miscarriage, but please let us know more of the dates and other symptoms you are experiencing.

Some women who experience prolonged periods may have hypermenorrhea, and you can read about this below:
From the EmpowHER site: Hypermenorrhea: "Menorrhagia is excessive and/or prolonged menstrual bleeding at regular intervals. Although about 30% of women have heavy periods, only 10% of women have menorrhagia. On average, menstrual flow occurs every 28 days and lasts for five days. Four tablespoons to one cup (60 to 250 milliliters) of blood is lost during this time."

The link above also tells you what tests you may expect at the doctor's office. Most likely a pap test, possibly a blood test. Be sure to ask lots of questions, and please let us know if you have any further questions before and/or after your appointment.