While doing any lifting in gym, don't hold your breath. Exhale when lifting the weight and inhale when lowering the weight.

Whenever you feel that the activity is new for you, you will concentrate more on doing it correctly. During this, you forget the natural things that you need to do. You need to consciously remind yourself to breathe. Holding your breath causes your blood pressure to rise. If you hold your breath for even a longer time, it may cause fainting.