If you say '설레' that means your heart flutters. 설레잖아 can be different depending on situation. What I learn from Korean movies is if you say it someone else, it's like a reason for you hesitate. A guy don't open the door and come in. So the other guy say "why don't you come in?(안 들어가고 뭐해?)" and than he says 설레잖아(I'm not ready). And the other guy say "you are not ready for what(뭐 때문에?)" and he just came in. I couldn't figure out better translation for 설레다 but obviously the guy wasn't ready but not for bad reasons (Actually this was due to the fact that there was a girl he love in that movie.) Or other situation, you talking your self or you just saying 설레잖아 it's like 'damn, my heart flutters'.