Some people seem to think their body and minds build types of defense mechanisms.. something that will go against your LD and cause you to wake up. If you've had lucids before though with no problem, I'd say that's probably not the case. Maybe you were so excited about going lucid that you got anxious which made it harder for you to breathe in the dream? My advice is to be chill when you go lucid and don't get too excited. Makes things more vivid and the overall experience lasts longer, at least for me.