I have a part welch corgi, part german shepard. I know that is a strange combination, but he was a puppy that no one wanted, so we took him in. He is 6 years old, and the best dog anyone could ask for. Tonight he came in from outside and was walking stiff legged, and slightly shaking. My wife thought he was having a heart attack, I checked his heartbeat, and it was fine. He was breathing normally too. This went on for 2 or 3 minutes then he was fine. He picked up his ball and brought to me to throw for him to retrieve. He seems fine now. Do you have any explanation or suggestion as to what may have been happening? Any help would be appreciated. The temperature is 55 degrees at the time of incident so cold was not a factor. Thanks.

Optional Information:
Age: 6; Male; Breed: welch corgi/german shepard mix

Already Tried:
nothing, just checked breathing and heart rate. Checked nose to see if it was cold and wet.