Last Saturday, I took Redford & Tucker to the dog park.  Redford likes to run up and down the fence line with the big dogs (we're on the small dog side).  I let him run until he was really tired. 

That night he was moving slowly.  I figured it was weekend warrior syndrome since we hadn't been to the park for a while.  He gets lots of sprinting in the yard, but not sustained running.

he always liked to sit near me when I sit on the couch and would shove Tucker or Butler out of the way.

Since then, if he is in the house, he is either in bed or in the kitchen on the wood floor.  If he is out, he won't come in.  He won't be affectionate inside, but is overjoyed if I sit outside with him.

He stays in bed until breakfast and then will only go out when he is made to go out.  He used to demand to go out immediately after a meal and then come right back in.  We called it checking to make sure outside was still there.

He doesn't have any hot spots or sensitive spots.  He's walking and running normally.  There's nothing on his paws. He's on medication to help keep him from joining in the fights when Butler picks on Tucker, but he's been on that for 60+ days.

I'm really at a loss as to what could be wrong.  There have been changes in the past months where I've had to separate Butler from Tucker & Redford because he's picking fights, but that's the only change.

Any thoughts?