Sorry to hear about this. Its highly unlikely to be cancer given your age. Best to get it checked out though - no harm in doing so even if just to ease the anxiety. Some of the symptoms could be anxiety related (e.g. rapid breathing and stomach pain).
I had on-going similar symptoms (pelvic pain, brown discharge, lower back pain, bloating) and I had a large benign ovarian cyst. This was found by my doctor who I went to for abdominal pain and she noticed my stomach felt very full. Ultrasound revealed cyst. Surgery was required.
Sorry for your troubles! If you do not have a family history of ovarian cancer, it is unlikely to be cancer since the average woman's lifetime risk is only 1.3%. Something is certainly going on since you have not had a period for 6 months. But some endocrine abnormalities (such as thyroid, pituitary, adrenal) can cause menstrual cycle irregularities. So can the metabolic disorder termed Polycystic Ovary Syndrome (PCOS). It would be a good idea to see your doctor (primary care or gyn) to get checked. Best of luck to you!