Not saying Don is wrong. But something else to look at is Tick paralysis. Seems to be bad this year. Some symptoms are what seems to be soreness.(laboring to try and get up, like back end isnt working, and holding back leg or legs up in air off ground while sitting. Dog will seem very lethargic and shows no signs of wanting to get up. Doxycycline for treatment and gatorade or pedialite for dehydration seem to help. Alot of kennels have had dogs go down hard this year with it. Not saying thats what your dog has, but check it over just in case.