First, at 100' you were probably somewhat narc'd. If you ascend a little, that usually takes care of that.
Also, what doesn't get as much mention, as you get deeper, your reg is feeding you air at higher and higher pressures. As the air gets more dense, it becomes a little harder to draw into your lungs. With today's high-quality regulators, usually the regulator isn't the problem, though some do breathe harder as you go deeper. But usually what gets interpreted as a regulator issue, is really just the fact that the air you are breathing is 5 times more dense that you usually breathe.
Also, what doesn't get as much mention, as you get deeper, your reg is feeding you air at higher and higher pressures. As the air gets more dense, it becomes a little harder to draw into your lungs. With today's high-quality regulators, usually the regulator isn't the problem, though some do breathe harder as you go deeper. But usually what gets interpreted as a regulator issue, is really just the fact that the air you are breathing is 5 times more dense that you usually breathe.