i had the exact same issue.. except when i took arty to the vet he couldnt figure anything out. The most we could think was that arty tried to sneak out the door as I opened it and the door hitting him might have bruised him. Thinking back, i think it could have been a sprained tail as well.
He would yelp when I tried to pick him up or sit him on his butt but then when i layed him flat and touched his stomach he was fine, and didnt mind however hard I pressed. but then when i picked him up he would yelp again. sometimes the pain would be so bad he would accidentally pee a little bit. He was also very sluggish that day but a few days later he was back to his old self again.
He would yelp when I tried to pick him up or sit him on his butt but then when i layed him flat and touched his stomach he was fine, and didnt mind however hard I pressed. but then when i picked him up he would yelp again. sometimes the pain would be so bad he would accidentally pee a little bit. He was also very sluggish that day but a few days later he was back to his old self again.
