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!
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.