i have that problem too, its ingrowing hairs as they start to grow back and irriation from the razor. I find shaving in the direction of hair growth (in pubic area in particularly) for a while until the sensitive skin becomes used to the razor, then starting to shave against the grain becomes alot easier. exfoliate your legs before you shave them, that helps prevent ingrowing hairs afterwards, always use fresh razors and i find tea tree really soothes irritation