Last month my period was very light-test was negative - what could be reason for this?

Anovulatory bleeding: Pregnancy is always on the list of possibilities when a woman has abnormal uterine bleeding or cycles. You may be having anovulatory cycles (cycle without producing an egg from the ovary). If a urine pregnancy test is negative, i would recommend evaluation by your gynecologist, and treatment will depend on the cause of the abnormal bleeding and your current short-term pregnancy goals.