I have a 3 year old Bernese female who has lost her control of her back legs and can't get up, her temp is 103.5 and her tail and legs are hot

Please tell me more about your Bernese.  I she spayed?  If you pinch the toes of her back legs can she feel it and respond by withdrawing her legs?  Be sure you watch for her head motion to be sure she really feels the pinch.  When you took her temperature was she bundled up in a blanket that could make her warmer?  Does she go outdoors or is she always inside?  Has she seen a veterinarian recently?

Please tell me more about your Bernese.  I she spayed?  If you pinch the toes of her back legs can she feel it and respond by withdrawing her legs?  Be sure you watch for her head motion to be sure she really feels the pinch.  When you took her temperature was she bundled up in a blanket that could make her warmer?  Does she go outdoors or is she always inside?  Has she seen a veterinarian recently?

I am assuming you are talking about a Bernese Mountain Dog.  I am also assuming that she can't get up at all due to her hind legs but can move her front legs OK.  Does she seem painful anywhere along her back or tail or her hips?  Are you able to take her to a veterinarian for evaluation?

There could be several reasons why she can't get up.  Her temperature at 102.4 is in the high normal range, especially if she is distressed at not being able to get up.  If her back is painful a disc may be rupturing in her back and pressing on her spinal cord.  That could make her tail seem limp though wouldn't usually cause the tail to be swollen.  This can come on quickly and is usually very painful.

Another possiblility is a fibrocartilaginous embolus.  This can be painful or not or painful then stop being painful.  This can look very much like a ruptured disc but the dog is much less painful.  This is treated with lots of nursing care in a dog as large as yours.  You would need to provide very padded bedding, turn her often and express her urinary bladder for her.  A water bed may work well.  It is not cured by medication or surgery that I am aware of.

If her head and forelegs are moving I this is unlikely tick paralysis.

I asked if she was spayed.  Sometimes an infected uterus can cause fever and weakness and even toxemia.  If she is eating and alert this is unlikely.

The breed is prone to hip dsyplasia.  If both hips flare up at the same time it could cause her to be unable to get up.  Does she have a history of hip problems?

The breed is prone to tearing the anterior cruciate ligament of the knee.  If she had ACL tears in both knees and was overactive one day, she may not be able to get up but should be able to mover her legs and tail.  Has she been having knee problems?

If you need help to get her up into the car or help with treatment there may be an animal rescue in your area who could help.  Placing a bathrobe belt under her stomach or a winter scarf then lifting each side is one method to lift her.

You mentioned that she is cephalexin antibiotic.  There are a few animals and people allergic to cephalosporins.  This doesn't exactly fit that type of reaction from what you describe.

Continue monitoring the temperature.  Organisms can cause meningitis or weakness though not usually just the rear part of the body.

That was a good job getting her to a veterinarian.  So let's get to work on treating her symptomatically.  You said you have access to Baytril. That would be about 136 mg for a 100 lb dog every 12 hours.  That would be a good broad spectrum antibiotic.  If they didn't see anything on X rays with the tail you need to start shaving it and the area of the base of the tail.  Just be agressive to get to where you can see the skin well.   We are looking for wounds or snake bite or anything unusual.  What type of flea and hearworm prevention is she on?  Let's try some benadryl.  The high dose would be one mg per lb of body weight every 8 hours.  This would be 4 of the 25 mg for a 100 lb dog.  

If she can't stand be sure to keep expressing her bladder and keep her very clean and dry and get the water bed.  Keep turning her.  

They should have seen signs of a pyometra on X rays and if it is bad enough to cause her to be unable to get up the blood CBC should have been abnormal.  Still, watch for any discharge from her vulva and the Baytril should help.  Be sure she is eating and drinking well.  Stop the Cephalexin.

I will look these over.  Do you know if they have a radiologist review their films?

Have you seen any progress?  I think that your regular veterinarian is trying to cover each possibility.  

Looking over th films on the screens you sent, I don't see an obvious cause for the hind leg paralysis and swollen tail.  An MRI may show more.  Spinal cord fibrocartilagenous infarct would not be seen on plain films as well as tumors of the cord and some injuries.  I see on one film that her urinary bladder is large.  There may be an enlarged uterus that is being masked by the bladder image since they both have similar densities and locations but I don't think that is likely.  As I said before, it is important that you help her empty her bladder several times a day to avoid sludging and irritaion of the inside walls as well as infection.

Have you been able to cut her hair?  The tail and also around her rectal area and vulva should be trimmed short.  

What medications are you giving.  What is her attitude?  Is she painful? Can she move at all or get up?  What has her temperature been?

Photos for me would be helpful.