I've had nexplanon for a year now and never had a period and now it's lasted a month with dark brown blood and clots is that normal?

Yes: Nexplanon is a progesterone only method of contraception and is associated with irregularly irregular bleeding which means there is no pattern to it. It is unpredictable . Estrogen is in contraceptives to regulate periods. Progesterone makes them lighter and provides the contraception. Still, a change can be indicative of infection and you should check with your doctor to be sure.