Dear get tests done and rule out the cause so that your disease can be nipped in bud.
some times causes are very minor and your fear makes it great and this fear becomes the cause for loss in weight.
Dr. Rajesh Lakhanpaul

The cause of gradual and rapid loss of body weight can be due to several reasons. You need to consult your Physician ASAP. However, in order for me to assist you further, you need to tell us more about following associated signs and symptoms, if any;
1. Is there is any associated low grade fever (evening rise), night sweats, long unexplained standing cough, long standing diarrhoea, diabetes, etc. You need to do few important laboratory investigations (biochemical tests, CBC, routine & microscopic tests for urine, stool, HIV/AIDS, etc) + x-ray of chest to rule out most common ailments that might be the cause of your weight loss.