Did you remember to breath? That is a pretty common thing. You get to punching and hold your breath, wears you out pretty quick. You see this in fights and they call it punching yourself out. Nope. A guy that has been training for a long time can punch all day, unless he forgets to breath. happens when you get excited or when you get tight.
19 year olds don't "over-train". As long as you are sleeping good and eating right. Monitor your resting heart rate if you are really paranoid about it.
i would try to incorporate a wider variety of drills into your routine. different bags and footwork drills.