It suddenly got extremely hard to get any air. The dive master thought i was going through air a lot quicker than i should have been. Never really got an exact reason. I assume it was a leak. Maybe a bad o ring or something, I'm assuming. He said he wasn't initially concerned, because i still had plenty of air when he asked for readings. Then I suddenly had trouble dragging any air in. As soon as I got his attention, he was on me with his octo. If more experienced, I probably would've known in the beginning something was off. Breathing wasn't as smooth and easy as it was on any previous dive. I just figured some regulators act differently. In hindsight, the problem was probably obvious and, being inexperienced, didn't pick up on it.