I would think that blood work might have been something to do. If he seems ok today and isn't vomiting, that is probably a good sign; but, of course if that changes, you need to get him back to the ER.
Hard to tell what the jerking was....if he does it again, try to video it.
I would definitely take him to his vet in am no matter what just to get him looked at....if you have not had blood work done on him in some time, you might ask your vet if it would be helpful. I get my pups bloodwork done at a minimum of once/year. As they age, usually every six months. That is where you usually see changes before they become major issues |