In otherwise healthy women your age, by far the most common cause of constant spotting, especially if you are not sexually active, is something called an anovulatory cycle. This is a phenomenon in which the ovaries fail to release on egg, which means that the hormonal trigger for the menstrual cycle doesn't occur. Instead, the thickened uterine lining, which is waiting to be shed in menstruation, becomes more and more friable, leading to small amount of daily bleeding
I recommend that you go to see your primary care doctor
or your OB GYN doctor about this issue. They will take a more detailed history from you and also perform a thorough physical examination, looking for any concerning signs or symptoms. If they confirm the likely diagnosis of an anovulatory cycle, then they will be able to give you a simple medication, called progesterone, to provoke a full period and reset your cycle. Another treatment option would be oral contraceptive pills, especially if this becomes a regular problem, as they can help to regulate your periods on a month to month basis.
Start by making an appointment to see your doctor at your earliest possible convenience; they should be able to help you out!