I had my period for 4 months and it's either light or normal never heavy i'm 16 and never had this kind of condition happen before so what could it be?

Irregular period: There are several reasons for an irregular period. If you are sexually active, the issues to consider are pregnancy and infection. These require getting tests. If you are not sexually active, period changes can include stress, hormone imbalance, weight changes, and non STD infections. To really evaluate the cause, it is best to see a doctor.

Vag bleeding x 4 mo: This amount of bleeding is abnormal, and you should discuss this with your parents to help arrange an appointment with your family doctor or gynecologist for an examination and diagnosis. Often when one bleeds this much for so long, they become anemic due to low iron. This also needs to be checked. If you have become sexually active, please discuss this with your doctor.