The two things that have symptoms similar to what you are describing are pancreatitis and a back injury of disc issue. Pancreatitis is an inflammation of the pancreas. The pancreas is a gland that produces and releases enzymes into the digestive track that aid in the digestion of food. When these same enzymes are activated while they are still in the pancreas, inflammation occurs and the enzyme start "digesting" the pancreas itself. Some common causes of pancreatitis are eating a lot of very fatty foods, obesity, steroid use and other diseases and conditions.
Symptoms of pancreatitis vary but include vomiting, diarrhea, anorexia, weakness or collapse, dehydration, shock, fever, depression, and upper abdominal pain. A dog may indicate abdominal pain by acting restless, panting, crying or wincing when picked up, shaking, standing with an arched back, or lying with the front end down and the rear end elevated
You can read about it in detail here:
Since you do not indicate any of the other symptoms of pancreatitis such as vomiting and diarrhea, I'd lean more toward a back problem. Dachshunds are prone to back injuries and Intervertebral Disc Disease. It's entirely possible has this condition or has injured his back. You can read more about this here:
An intervertebral disc that has slipped or ruptured up into the spinal canal causes inflammation of the spinal cord, which in severe cases causes paralyses of the rear legs. You can read about this here:
Since anal gland issues can cause various symptoms in a dog, it would be a good idea to check those as well. Some dogs with full anal glands will tuck the tail and walk funny. 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. Excellent site on anal glands
Arthritis might be an issue as well, though usually artrhitis is worse when they first start moving around, so I think you need to get in to see your vet and have your dog checked. As I'm sure you know, you will probably need x-rays to check the discs.
I hope this information is helpful to you.