Did you give a stool sample? It could be a tapeworm or another parasite, they eat all your food and still be symptomless.
Good luck!
First things that come to my mind are hyperthyroidism and diabetes. Many times blood tests come back in a "normal" range, but there can still be symptoms.
Have you changed the types of foods you eat, or been getting more physical exercise? Lots of stress?
I don't believe it's diabetes or a tapeworm, it might be nothing. My diet did change a lot last year, i was working out a lot and eating low fat. And then this year I was dealing with a lot of stress.
If you drastically changed your diet or worked out more, that could account for the weight loss. Being under a lot of stress could keep you from gaining weight back.
Otherwise, it might not be a bad idea to get your thyroid checked out anyway.