Tentatively, it sounds to me as though the temperature spiked in his cage, resulting in a regurge. It is hard to be 100% certain without being there, perhaps he just freaked out due to the thunderstorm and barfed it up, maybe he had a stomach issue, to be honest with just one regurgitation and nothing really clear cut... Could have been a few things.

From here, give it at least a week for its stomach to settle down and relax. I would offer it a meal somewhat smaller than usual, and then give it a full week and see how it does. From there, if its still keeping food down, go ahead and go back to your usual routine and size of prey items, and consider it a fluke. Personally, I'd keep it in quarantine for an extra few weeks, just in case... Just to make sure it doesn't happen again.

And count yourself fortunate! This kind of thing is exactly why quarantine is important. Now you know to watch the snake and make sure it was just a one time occurrence, not the start of a bigger problem.

-Jen