Boy what a scare that was. I am glad he is much better now. Yes, sometimes dogs can just get sick but it is fairly uncommon. Most of the time there is another problem that causes a dog to become ill. I sounds like he maybe ate something he should not of or possibly got poisoned. Some vomiting is ok, but what you described about him having trouble walking is not normal. Keep a good eye on him & I hope he still feels better.