An unexplained drop in weight could be caused by various conditions including overactive thyroid (hyperthyroidism), diabetes, depression, liver disease, cancer or disorders that interfere with how your body absorbs nutrients (malabsorption disorders)...Consult nearest physician for detailed evaluation with all reports of investigations they will do the needful.

Since you have done PETscan too....it means the doctor must have ruled out the probable causes...need to see the investigations done till now....but we often miss infective causes of which most important is TB....and nowadays HIV infection......look whether they have been excluded