She shakes, tenses up and is very sensitive to touch.

How old is the dog? Is she healthy otherwise and eating, drinking and using bathroom okay?

It sounds as if she may be reacting to pain or discomfort. Of course I am only guessing, but if she is not acting herself she should really be checked by a vet to pinpoint the problem.

She is a 4 year old Jack Russell. I had her at the vet two days ago because she would yelp if I toughed her side. He said she was constipated and put her on soft food diet. She is pooping. Eating and drinking. Today she gave her "I can't get down" bark. When I sat down beside her she was all tensed up and shaking. I tried to pet her and she yelped. A while later she was fine. This has happened twice.

She is a 4 year old Jack Russell. I had her at the vet two days ago because she would yelp if I toughed her side. He said she was constipated and put her on soft food diet. She is pooping. Eating and drinking. Today she gave her "I can't get down" bark. When I sat down beside her she was all tensed up and shaking. I tried to pet her and she yelped. A while later she was fine. This has happened twice.

I would call your vet and see what they say. It does sound like pain and could be related to the constipation.

I agree, the shaking is her reaction to pain. Could be an injury you did not witness, any other pets in the home or small kids?

How did they know she was constipated? Did they x-ray her? Or ultrasound her? Never settle for just a guess... If there is nothing but words to back up constipation, then you need to go to a different vet.

She could have a blockage in her gut that is trying to push through. That is painful! It may not even be a blockage, she could just have severe back pain or abdominal pain. When dogs don't want to lay down, its typically the back or abdomen.

Call a different vet... its always good to get a second opinion anyway .