I am sorry to hear that your pup Pepper seems to not be her usual energetic, perky self for the past 4 days, difficulty going up and down the stairs.
I am glad she eating and drinking.
Is she limping, or does she seem to be walking/moving more gingerly than usual?
It sounds like she has some musculoskeletal pain that is making her feel lethargic and reluctant to do the things she usually easily does.
There are several reasons for lameness/musculoskeletal pain
Dogs that are painful will hesitate jumping or climbing stairs and display the sort of behavior she is, either because it hurts to do the activity or because they feel weaker than usual. They tend to sleep more because they are painful when they are up and moving around, and it can be exhausting guarding themselves, tensing their muscles, and having to work harder to do what used to be so much easier.
This is most commonly associated with a problem with their intervertebral discs, which are the spongy cushions between the individual vertebrae in their back and neck. These spongy discs can move or rupture and place pressure upon the spinal cord which can lead to pain, and in severe cases paralysis.
Radiographs can sometimes be diagnostic but often early on in the disease process, because the discs are soft tissue not bone, everything will look normal. An MRI is the best way of diagnosing disc disease.
If the dog is painful but has no evidence of paralysis we can try strict rest, anti-inflammatories and pain medications for several weeks to allow healing.
If there is evidence or weakness or paralysis then surgery by a board certified veterinary neurologist, as soon as possible, is indicated.
Ideally she would see her veterinarian. If this is indeed a disc problem your veterinarian can prescribe a steroid or nonsteroidal anti-inflammatory to relieve pressure on her spinal cord and nerve roots, as well as something for pain too, such as Tramadol. And if she is having painful muscle spasms then a muscle relaxant such as methocarbamol as well.
She should be closely confined starting now. No stairs, running or jumping. If you have a crate for her I highly recommend using it. The less she moves around the more comfortable she will be and the faster she will heal. She should go out on a leash to relieve herself. Do not use a collar for her, a harness which more evenly distributes forces if she pulls on his leash is better. You will need to confine her for several weeks, even as she starts to feel better or she may reinjure herself.
You can alternate warm and cold compresses on her back and neck muscles for 10 minutes at a time several times a day. Cold reduces inflammation, and heat helps soothe painful muscle spasms.
Keeping her on the thin side is recommended to decrease stress on her back, but is no guarantee that she won't have another episode. Once a dog has one bad disc the likelihood of another is very high.
If you are interested in reading more here is a link to an excellent article about intervertebral disc disease, its causes and therapy: http://www.petwave.com/Dogs/Dog-Health-Center/Bone-Joint-Muscle-Disorders/Intervertebral-Disk-Disease/Symptoms.aspx
There are other less common causes of back pain such as infections, tumors of the vertebrae or the spinal cord itself or fibrocartilagenous emboli but far and away disc disease is the most common cause of back pain in dogs.
If she doesn't seem especially painful it may be that she has some sort of internal organ or endocrine disease that is making her feel very sluggish. Ideally she would have some screening bloodwork done, a complete blood count, biochemistry profile and thyroid profile if she seems more weak/tired then painful.
Please let me know if you have any further questions.