Secondary amenorrhea, or the interruption and absence of menstrual flow in a previously menstruating female for at at least cycle intervals or six months, may have a number of possible underlying causes. Primarily and in sexually active females, pregnancy must be rules out: if you suspect being pregnant and you're not sure about the test you took, you need to be examined by a physician to be sure, including an ultrasound of the uterus for documentation.
A thorough evaluation will be in order once pregnancy is rules out: we must look for problems in the endocrine glands, starting from the pituitary gland (the pine-like small gland in the brain that controls the function of other glands, like the thyroid gland and the ovaries), the hypothalamus (another important gland in the brain with major endocrine functions), a review of any medications you are using, especially long-term use, and of course we must assess for any sharp drop in weight (leading to a significant reduction in energy levels and starvation mode).
Also, a detailed physical exam should be performed, and look for any abnormal nipple discharge (indicates pituitary gland disorder), or abnormal hair growth.
You need to visit the doctor's office to proceed with the necessary evaluation and manage accordingly.