Hello,
Welcome to the forum and I am really glad to help you out. Your concern is regarding the absence of periods or amenorrhea that you are having. If serum HCG test is negative then it is unlikely that you are pregnant. However confirmation can be done with an ultrasound uterus.
If pregnancy is ruled out, then other causes of delayed periods need to be looked into. It can happen because of hormonal imbalance, increased stress, eating disorder, increased smoking, alcohol or caffeine use and medical conditions like PCOS, fibroid, polyps and endometriosis. So my suggestion would be to consult a gynecologist and get an ultrasound uterus done. Not only it clears the status of pregnancy but also can help in diagnosing medical conditions causing delayed periods.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care.