If you have an adrenal tumor, does weight loss signify development of cancer?

Not usually. Assuming that you have had a ct showing a mass in your adrenal it should be worked up to make sure that it is not producing hormones and has no radiographic signs of cancer (and is not more than 5cm in size). If all of these are ok, then it is a benign "incidentaloma" and may be followed with ct. Your weight loss should not be related unless you have another cancer metastatic to adrenal (unlikely).
Concerning. If there is a sudden rapid weightloss it is concerning that there is a problem. Whether it is cancerous can only be identified by further testing - you might need to see a specialist in this area.