I'm sorry to hear about ChoCho's hesitancy to jump and refusal to dance on her rear legs as she usually would, along with nipping when her flank is touched.
She sounds uncomfortable.
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.
It sounds like she is having back pain, in particular trouble with the intervertebral discs in her back, 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, weakness, muscle spasms, 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 now since this has already persisted for a week. 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.
In the meantime 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 her leash is better. You will need to confine her for several weeks, even as she starts to feel better or she may reinjure herself. 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.
Another possibility for rear leg lameness/weakness in a small breed dog is a luxating patella (knee cap that pops in and out of place, or stays out of place). But that should not cause flank sensitivity and tends to be more chronic conditions, and symptoms often become apparent at a younger age.
With a patellar luxation the kneecap slides out place rather then staying in the patellar groove. When that occurs the leg cannot bend or flex as it should and the dog becomes lame. In mild cases some dogs learn to kick their rear leg and pop the knee cap back into place. Over time however because of the constant wear and rubbing of the knee cap moving in and out of place we see secondary arthritis forming. If she has had on and off lameness in her rear legs then this is possible, just less likely with her history.
At this point I do recommend a veterinary exam because this has persisted without improvement, and if this is related to disc disease then anti-inflammatories should be prescribed to prevent things from getting worse.
In the meantime keep her quiet and do not encourage running or going up and down stairs.
Please let me know if you have any further questions.