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It may be that he injured his tail, so you will want to check his tail over well. He could also have a problem with his anal glands which will cause a dog to hold their tail down as well. The anal glands are 2 sacs on either side of the anus at about the 4' o'clock and 8 o'clock positions. If your dog is continually licking their anus and dragging it, it could also indicate that they are full. This problem can usually be resolved by emptying out the glands. To empty them or express them, you will want to cover the area with a tissue and press your finger on the dog's anal glands with an upward motion. A foul smelling liquid should come out. If nothing comes out and your dog appears to be in pain, you should take him to the Vet. Your vet can do this easily, as he is experienced at it. A dog will often avoid jumping as it stretches the skin over the glands and is uncomfortable. Excellent site on anal glands
There is a condition "cold tail" that Labradors, English Pointers, English Setters, Foxhounds and Beagles sometimes get which causes them to hold their tail close to the body. You can read about it here.
It is possible that this is what you are seeing with your dog. With cold tail it sometimes takes up to 2 weeks for the tail to recover. Some owners feel the administration of anti-inflammatory medication aid in a faster recovery.
Dog's have sebaceous glands and perianal glands at the base of their tails. Sometimes, these glands can become clogged and swollen. A warm compress on the spot and cleaning with Benzyl peroxide gel may help with the problem. I would also use some Neosporin or triple antibiotic cream on the area as well. Dogs with this swelling of the sebaceous gland may have a problem with hypothyroidism or a hormonal imbalance, so I would suggest you have your dog seen by your vet if you do not see improvement.
Now another possibility which is highly possible with a chihuahua is a disc issue. This often causes a dog to avoid jumping and stairs. It can cause them to carry their tail down and walk slower as they ar not sure exactly what movement causes the pain. An intervertebral disc that has slipped or ruptured up into the spinal canal causes inflammation of the spinal cord, which in severe cases even causes paralyses of the rear legs. You can read about this here:
Check his glands first and if they are ok, I'd go ahead and have him checked over by your vet so an appropriate antiinflammatory medication can be prescribed for him. If it is a disc issue, I recommend strict crate rest for about a week as well to give the disc time to heal.
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