Hello, Welcome to HCM. I understand your concern. There are couple of reasons for rapid weight gain like Cushing s syndrome, hypothyroidism, diabetes, depression or obesity. Although you have not mentioned you sex or other medical or illness history, I think you should first rule out cushing s syndrome and hypothyroidism. If you observe change in your like moon face, hairsuitism, excess fat deposition on shoulders, back of neck and around your belly than you should get tested for cushing s syndrome. Cushing s syndrome is due to excess of steroid production in body. If you are feeling low mood, fatigue, edema of leg and slow heart rate, than there is possibility of hypothyroidism. Thyroid function test can easily detect presence of hypothyroidism. If you feel frequency of urination along with excessive thirst and appetite, than you should get your blood sugar tested for diabetes. If nothing of these is present, than you should look for your dietary intake. Many times an individual feels that he is not eating much but that may not the case. You should look for energy dense foods in your diet like cheese, butter, cream containing foods and replace them significantly with vegetables, salads and fibers. Hope this will be helpful. Take care.