Hello, my name isXXXXX and I have over 20 years of experience as a veterinarian. I am very sorry to hear about Purrsian's reaction to touching her back. The video was quite helpful and points to either pain or extreme itchiness given her reaction.
Allergies are the most comon cause of itchy, uncomfortable skin. And flea allergy is the most common allergen. It seems to affect the tailhead and back area most so it is definitely possible.
I'd use flea protection every 28 days. Both Advantage II and Frontline Plus work quite well at killing fleas and are less likely to create a skin reaction. I do not recommend using any of the over the counter flea products as they are ineffective and some can be toxic.
Other allergens are inhaled (like mold spores or pollens) or food allergies.
To treat her allergic reaction to the flea bites (or any allergy) I recommend
antihistamines and omega-3 fatty acids. Some antihistamines to try are:
1) Benadryl at 1mg per pound or 1/2 of a 25mg tablet per 8 to 15 pound cat 2 to 3
times daily. You can crush the pill up and hide it in something tasty like her
canned cat food or you can give it directly. Make sure whatever you use is Benadryl only as the combination products with decongestants and acetaminophen are toxic to cats.
2) Chlorpheniramine 4mg per cat once or twice daily.
The omega-3 fatty acids will work synergistically with the antihistamine to soothe
her itch and as a bonus will improve her hair coat and skin condition in general. I like the products by Derm Caps or 3V. These come in pump form to put on the food or you can puncture the capsules and dribble it on her food.
At her age spinal arthritis is also a definite possibility.
Long term for joint pain I do recommend using a combination of a glucosamine/chondroitin product (examples are Dasuquin or Cosequin) and an omega 3 fatty acid (like 3V Caps or Derm Caps). These work synergistically and
improve cartilage health and joint fluid quality and quantity as well as reducing inflammation. They can take several weeks to see full improvement but some cats do very well with them alone. They are available over the counter.
Another option is a product called Duralactin. This is an anti-inflammatory product derived from milk proteins and it also has omega 3 fatty acids incorporated into it which can be very helpful. See this link for further information: http://www.duralactin.com/products_feline.htm
Another possibility for her discomfort is a condition called hyperesthesia.
Hyperesthesia is a poorly understood syndrome which is difficult to diagnose.
These cats have pain and hypersensitivity to touch on their back and tails.
They sometimes will bite at themselves because they are so uncomfortable. It is
a poorly understood disease process with no definitive test for diagnosis and so other problems (such as spinal arthritis or infections or allergic reactions) should be ruled out first. We do know that stress does seem to make this disease process worse or flare more frequently. Sometimes mood altering drugs and steroids can be helpful.
Here are a couple links discussing hyperesthesia syndrome if you are interested: http://www.veterinarypartner.com/Content.plx?P=A&A=1998&S=4&SourceID=62
Another condition which intially causes pain, but can later lead to urine and stool
incontinence is lumbosacral stenosis. This is a narrowing of the bony canal that the spinal cord travels through, either due to weak ligaments holding the vertebrae in place or arthritis changes in the vertebrae. It is more common in dogs than cats but it can occur in cats.
Has she has ever had radiographs of her spine? I think those would be quite helpful.
Finally it is possible that she has a slipped intervertebral disc (the soft cushions
between the vertebrae which allow spinal flexibility). When this happens pressure is placed on the spinal cord which is quite painful.
If she goes outdoors she may have had some sort of trauma where her tail got
caught and the nerves were stretched. As the nerves regnerate there is a tingling, sometimes painful sensation which can lead to the cat chewing or biting or being very sensitive at the area.
LS stenosis and a slipped disc can be diagnosed with regular radiographs sometimes but many times an MRI or myelogram (dye study of the spinal cord space) is needed.
Anyway in your cat's case I would start with trying to control allergies and if that
doesn't work then she needs a thorough examination, as well as bloodwork to make sure internal organ function is normal, and a urinalysis with culture to look for a urinary tract infection, as well as radiographs of her pelvis/spine.
If that all looks normal referral to a neurologist for an MRI may give you the
answers that you need.
Let me know if you have any further questions.