Unfortunately, with this little information, I can only give you some possible causes for the pain and swelling. It may be that she injured her tail, so you will want to check her tail over well. If she is trying to bite you, you might need to put a temporary muzzle on her. You can see the following site to see how to fashion a temporary muzzle which will let you examine the tail region.
She could also have a problem with her anal glands which will cause a dog to hold their tail down as well. If the swelling is bad, the tail might also be affected. The anal glands are 2 sacs on either side of the anus at about the 4' o'clock and 8 o'clock positions. A dog with an anal gland issue often licks their anus and will drag it. Those symptoms could also indicate that they are full. If they are full, that 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, you should take her to the Vet. Your vet can do this easily, as he is experienced at it.
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. Other breeds can also develop this as well. 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 aids in a faster recovery.
Dogs also 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.
Pain in this area can cause a dog to not want to move around much and that is likely the reason why she isn't moving too much. However, dachshunds are prone to disc problems which can cause paralysis and trouble with the rear legs, so you do need to keep that in mind as well.
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Since there have been recalls on certain foods, please check the following site to be sure the food your animals eat is not affected. If it is affected, contact your vet as soon as possible. Have your dog seen if they have any symptoms.