sounds very familiar to me, and i think i can explain what it is too.

when we live a life of unhappiness (due to things out of our control) we grow used to it, and its what we expect out of life, to be unhappy. but when we actual feel happy, genuinely happy, we dont believe it, and our unhappy side of us refuses to belive this new happiness, so it refuses it, and makes you feel it is fake. it is because we live for so long being unhappy, that happiness seems fake, and that we dont deserve it. you do deserve it, and it is real. so next time you feel happy, just embrace it for what it is, nothing more, nothing less. and see how it goes.

simon