Is she spayed? Sometimes intact females have a difficult time keeping weight on, especially if they are active and run around a lot. If she isn't spayed, have her spayed and I think you'll see a difference in how she holds her weight.
Ginger also had a good suggestion about having her thyroid checked.
A trip to the vet for a general examination and a quick blood panel should tell him all he needs to know. Please let us know how things go.
Thyroid springs to mind when there is weight loss without loss of appetite. Maybe you could get her checked for hyperthyroidism? But there are other problems which could cause this. I would say see the vet about it, especially as her stool is not always normal.
Sometimes dogs who exercise a lot lose a little weight, because the calorie output exceeds dietary intake. Or if it is very cold weather, that can make a difference to how much energy is used up (ie how many calories) But if this is not happening to your other dogs, then it would be unusual for it to happen only to your Husky.
The best thing, I think, is to get her checked out at the vet.