BPD is about having extreme uncontrollable emotional responses to things, and extreme swings. You might at one moment react totally normally to something, and then that same day 2 hours later work yourself up into an uncontrollable rage about that thing. You often feel insanely depressed and think the entire world is against you, you isolate yourself, etc. People don't know how to deal with your swings either way, so whether you isolate yourself you will likely become isolated eventually and destroy your relationships.
Is occasional narcissistic behavior a part of BPD? Yes, but it is very different from what most people think of as narcissism as you would spend far more of your time in a really low swing sort of hating yourself.