She was fine, she had to lay off walking for a while. I had to stay home and take care of her, take her downstairs to pee/poop, etc.
Lasted a week.
Xrays showed a slight fracture in her CCL, but surgery wasn't going to do anything.
Rest.
I'm holding out hope that this is it. He's a super athletic dog and he's had injuries before but never this bad. I just think he would be in more pain if he tore something? He's so tough though so I don't know.
$100 says he was running and catching a tennis ball, and he hurt himself. Evidently (as per the vet) at least cocker spaniels aren't supposed to do that (but c'mon, they do it anyway). Mine was particularly famous for doing 5-6 rolls with the ball when she caught it, severely hurting (continuously) her joints.
We honestly had to evaluate how much fun we were depriving her. We never took ALL the tennis balls away. Because shit, she loved them. We did minimize playing fetch long distances.
This might sound a little hippy dippy but the same thing happened to my bro's dog very suddenly. It was a pinched nerve. The vet said she had to stay in her crate for like a month and take medication to heal. They took her to an animal chiropractor and she was back to her normal self the same day. Hope all is well soon!