you know how sometimes no matter how positive a thinker you are, no matter how strongly you believe in the things you do or say, no matter how truthfully you choose to live your life, sometimes negative energy gets in anyway? it creeps through the barriers, seeps in from the world around you, breaks into your heart little by little.
this was happening to me for a little while, and it's so frustrating to feel what's happening, know in your head that it's a bunch of b.s. that is just rooting around for your achilles' heel, and still not really be able to completely control it.
something about visualized meditation helps me a lot, meaning while in a meditative posture i do deep breathing stuff~in with the light, out with the dark. and i think that, combined with giving myself time to feel the stuff, deal with it, and decide to expel it is what does the trick for me. unfortunately you can't always just plop down in the middle of your life and blow the black energy from your body.
anyway... get to it as soon as possible though, really. the people around you will thank you. :)