where you lifting when you felt it being loose?

if yes, which exercise?

First off, do you have any history with shoulder injuries?

If no, have you been working out your shoulders everyday?

If still no, you have bad form.

I'm a newbie too... i think i know what you said (especially when using dumbbell)

actually it is not 'loose', but the muscle on left side is weaker than right side
and also your left hand is not able to control the muscle as good as right hand (i believe you are right-hander like me)

i do feel it when i start, now the situation is better when muscle is built up