Have a dog who was perfectly healthy from what I could see. Lost all her weight over 4 week period. But don't know why. But I will tell you what I've done so far.

Not worms, she's been wormed and stool sample done
Checked for heart worms, not that.
She eats and drinks like normal
Gums are pink, her stool is fine also.
Waiting on cbc to come from the lab