Why Has My 3 Year Old Jack Russell Terrier Been Suddenly Regurgitating His Food?

My Jack Russell Terrier has been regurgitating his food now for 3 days. He will regurgitate undigested food. He will suddenly throw up with no warning sometimes up to 5 times daily. Right before he regurgitates his food, he will get very lethargic, but then when he's finished, it's as if it never happened. He's healthy otherwise, hasn't had any problems. He still plays and has an appetite, but his food comes up soon after eating. This just came on him suddenly.