My name is***** and I am sorry to hear that Sadie isn't feeing well. Based on your history, I suspect that Sadie is painful somewhere. Another possibility is breathing difficulties. It is more comfortable to breathe standing up.
Is it possible that she injured herself in someway? Do you have any other dogs who she wrestles with? How long has been acting this way? Any change in urination or defecation? Eating and drinking normally? Is she laying funny inside? Can she curl up in a ball to sleep like normal dogs do? Any change in breathing? Does her breathing appear rapid or labored?
Can you ask her to stand square on all four paws and then rotate her head side to side? A normal dog should be able to twist their nose all the way around to touch the side of their belly. Next can you stretch her front legs as far forward as they possibly can? A normal dog should be able to stretch their front legs so far forward to be almost parallel with the spine. Next, put very firm pressure down her spine and note is she twitches, tenses or collapses. Pit bulls are very stoic and strong dogs, so you will need to press very firmly to elicit a pain response if it is there. Lastly, extend her rear legs one at a time out behind her. A normal dog should again be very flexible and allow extension to almost parallel with the spine.
Once that is done, you should also check her body temperature to ensure that her lethargy isn't due to fever. Normal body temperature for dogs is 100.5 to 102.5.
For tonight- if her temperature is normal and the movement of her joints and neck is unremarkable, I think you can safely wait until morning to see your regular veterinarian. They will perform an orthopedic exam and possibly recommend blood work, a urinalysis and x-rays.
If, tonight, she has a fever, you find a sore place on her body, there is vomiting or diarrhea or she unable to walk at all, I would strongly consider taking her to your local animal emergency room as soon as possible.
I hope this answer helps you. Feel free to contact me if you have any further questions.
Kansas State University
College of Veterinary Medicine Class of 2001