Here are some other symptoms of a Respiratory Infection to check for:
-Wheezing, popping, or gurgling when breathing. It will sound almost the same as a person with Bronchitis.
-Bubbles or mucus in and/or around the mouth or nostrils.
-Stargazing.The snake will keep it's head elevated for long periods of time to try and keep it's airway clear.
-Excessive yawning and face rubbing. The snake will yawn frequently and/or rub it's face around the enclosure to clear mucus from it's airway, mouth and nostrils.
-Look for dried mucus around the enclosure and on the furnishings in the enclosure.

Hopefully it is just a piece of stuck skin, as the others have said. Good Luck!!