You got the short story, thatās why 
Last Thursday night I noticed he kind of missed when jumping up on a 3" table and had a struggle. Seemed to be walking odd and tail carried low.
Friday he was grouchy and seemed to be in some sort of pain when he jumped, did not seem to want to sit on his butt at all. Low tail carriage. I also caught him straining in the potty box
Saturday noticed he was peeing just fine (I had been watching his potty box trips due to the straining I saw Friday night) but then straining after peeing. Called the vet who advised just to lube him up to see if it helped possible constipation. He also threw up. I started also giving him water through a syringe to stave off dehydration from lack of wanting to drink. He was very growly and grouchy.
Sunday he was even more in pain acting, and still straining and acting super sore about his hind end. Kept watering him, and still peeing just fine but no poop. He was up crying most Sunday night off and on, every time he went to sit or really do more than walk it seemed.
Monday he went in to the vets. They palpated him, nothing. Xrays had nothing. Blood test nothing. Urine test nothing.
Tuesday they redid xrays because he wiggled a lot the first go round and they were looking for a blockage in the small intestine, sent the new ones to a specialist who said no blockage. Both vets and the specialist are stumped. Cat is obviously still in pain. Tuesday afternoon I happened to ask the vet about his tail, if it had moved. He hung up and went to see, nope, no movement or even feeling below about 2" from the base. Looked at the xrays and could see a small separation in one of the vertebrae by the base. No one had been looking at his tail because they were looking at intestinal issues due to the straining. Vet said this was usually caused by being stepped on or being slammed in the door, neither which has happened to my cat. My only guess is he was rough housing with the other cat?
Tuesday picked him up, vet had given him antibiotics (serea or seriam or something), and said if there is no recovery of the tail in 3-4 days he will need it amputated. Heās also on a pretty strong opiate pain killer 3x a day.
Today he is the happiest (due to meds I am sure!!) I have seen him since Wednesday, but still no tail movement. I am giving it until Friday.