Hello, I have noticed that one of my Guinea pigs makes a coughing sound in the middle of the night. Their bedding is paper so it can't be dust. I have never heard him cough during the day and it confuses me. My room gets cold at night but I do cover up his cage with two blankets. He's also drinking and eating regularly and is always running and Popcorning and he has no discharge from his nose and his eyes aren't crusty. What could be the reason for this?