It's really important that you take the advice proffered above. Your dog is prime for "Bloat". This is an often fatal malady affecting larger breeds of dogs, especially those with deep chests... classic Lab description. Bloat occurs most often when the dog has recently eaten and then plays or otherwise exercises vigorously. The dog's stomach is unsupported within the intestinal cavity immediately behind the chest area. When the stomach is full and heavy the larger breed of dog can actually flip the stomach over and the esophagus and lower intestinal outlet from the stomach wraps around the stomach cutting off not only digestion but also blood supply... fatal if not treated within 24 to 48 hours. Vomiting or attempting to vomit is a diagnostic assessment well known among Vets. The Vet can easily manipulate the dog's stomach without surgery... usually. Don't delay, please...