My period has lasted for about 10 days, what is wrong?

It is very light but persistent and hasn't even browned yet. I have never been on birth control and I'm not on it now. I don't know if it's stress or something more serious. I'm 18 and have had my period for 3 years. It has always come every month for about 5-7 days and is always moderate blood. I am at a perfect weight and am pretty active. I have a healthy life, no stds but haven't checked for pregnancy. I doubt I am, considering the lengthened period.