One of mine did this once after a particularly energetic play session. It scared me a bit but I was playing him *really* hard and it was really warm in the room. I let him cool down (I even opened the freezer for him to lie in front of!) and have not played him quite so hard since and it’s never happened again. I did look it up and it's normal if it's happened after exercise, but as mentioned above if they do it without any exertion or reason, get to your vets right away!