Hi there...sorry about the delay. Dr. K. just asked if I could jump in here as she is having some computer problems this afternoon.
It's going to be hard to say for sure what is going on with your dog without actually seeing him, but I can give you a few ideas.
The heavy breathing and the shakes could be due to many things. It is possible that he has some pain somewhere. You mentioned that he was walking a littly stiffly. It may be possible that he has some back pain. It is quite common for dogs to have problems with the discs in between their vertebrae. Sometimes we can tell by standing the dog on all 4 and then feeling down each of the vertebrae to look for signs of pain. However, many labs will do all they can to hide pain so this may be hard for you to diagnose.
The next possibility is that there is a problem with his heart. There are many kinds of heart disease that can affect dogs. Sometimes, with heart disease we can see something called a "syncopal episode" and this can be similar to fainting or weakness. This will be hard for you to diagnose at home. Most dogs with heart problems will have the occasional cough as well...so if you are seeing this, this makes heart disease more likely.
The next possibility is that there is some bleeding somewhere inside of his system. This is why Dr. K. was asking you about the colour of his gums. Often we can see that the gums are pale if there is bleeding, but sometimes this can be hard to determine especially if you are not used to looking at gum colour. Some dogs as they get older can have a tumor or multiple tumors on the spleen. Sometimes if one of these ruptures we can get sudden blood loss into the abdomen which can cause shaking, weakness, respiratory distress and a high heart rate.
It will be hard to say whether your dog should be seen on emergency or not, but if he were my dog I would likely have him seen today. If this is a spleen tumor and we catch things early enough he can have the tumor removed. Some of these tumors are benign and once removed the dogs do well.
Your vet will do a good thorough exam and will likely do some xrays and bloodwork to determine what the problem is.
I hope this information helps! If you have more concerns, just hit reply. Or, if my answer is sufficient, then please hit accept. Thanks!