Could be possible that you didn't pick a spot. Often when you are used to aiming at a target with a bullzeye and you don't pick a spot at an animal, the arrow flies way off.

But that might not be the problem because you only shot low. Usually not picking a spot will result in a way off to the side, high or low shot. Simply shooting under the animal could be caused by many things.

To add to Jeepnut's ideas, your peep sight could have slid down, you could have flat plain missed, or you could have hit a blade of grass or an obstacle in the field.

Did you test your broadheads? I hope your broadhead isn't heavier than your field points. Maybe your rest is slightly low or your nock is too high resulting in broadheads flying low.

You probably just flat plain missed. It happenes to the best.

Nic