He may have been getting ready to pass a urate or poop. Just keep an eye on it.

What are you using to treat mites? You might consider getting PAM (Prevent-A-Mite) online and treating all your enclosures just to be safe. That stuff works the best. And, it's not bad to have around in case you have an outbreak later.

Just keep an eye on him for bubbles, mucus, whistling, etc. If he has the start of a RI, make sure he goes to the vet and gets treated ASAP to reduce your risk of passing it onto your other snakes. Also, just do simple things like washing your hands before/after handling any of your snakes, use separate feeding tongs, etc. Just try to keep things as clean as possible and hope he doesn't have anything contagious.