I have a Brittany Spaniel,she is 8 years old. She sleep a lot, still eats. But have loss a lot of weight. You can see her ribs on on back.

Time for a vet check.

Sudden weight loss can be a symptom of many things, from thyroid problems, to parasites to disease. There are too many for us to guess, you will need to get the vet to do an examination and possibly bloodwork.
When you make an appointment take in a stool sample as well to save yourself some time. Just get a specimen jar from the vets the day before.

A brittany spaniels life span is around 12-14, but that all depends on what you feed your dog and how much exercise she gets. Try switching pet foods to maybe a holistic pet food such as Canidae, Origin, Go, Now, Summit, Legacy or other such brands. They generally have better ingredients and therefore will be better for your dog. Seeing the vet wouldn't be a bad idea either, but I'm guessing you just want to get ideas right now ^_-

I'm not sure but you should check with your vet. Check her teeth, she may have some dental issues thast are hurting her. Maybe she is getting more exersice? She could have eaten something that messed up her stomach. I would bring a stool sample to the vet like the person before me suggested.
Check out this link for info on your dog. It has a hight and weight section too.

Brittany Spaniel Information and Pictures, Brittany Spaniels

I agree with shazzy. Sudden weight loss is a serious issue. Being lethargic is even a bigger problem.

There could be something brewing inside. Get her checked by a vet. She should go in anyway for a senior exam.