I have three young sebbies who are being raised together. They are 5 weeks old. Today, one of them was laying down with his neck stretched out. I walked out and he ran off, like normal. Tonight, he is tripping and falling, falling onto his back and he cannot get up. When he is in a pool of water, he swims and looks normal. When he walks, he stumbles and falls. He will eat and drink water. The others act fine. The heat here today was 100 degrees, but he had lots of shade and water. Any idea what to do? What is wrong? He does go up and down on the patio, which is about four inches off the ground.