What could be causing my extreme weight loss?

In the last 18 months I've lost 30% of my body m (65lbs) for no reason. I've become less active and haven't changed my eating habits. I'm becoming so weak that I can't do many aspects of my job in retail. Because of the weight loss I'm hypoglycemic and had to up the protein in my diet significantly. Even that didn't help. The endocrinologist found no problems in blood work. Help!