''I'm stressed out,'' confesses Sigel on The B. Coming, his third album. You can't blame the rugged MC for being a tad overwhelmed. Currently serving a yearlong prison sentence on gun charges, he was clearly inspired to balance his usual crack-'n'-gats talk with unvarnished self-reflection. From the low-key paranoia of ''Feel It in the Air'' to the optimistic, Philly-soul-sounding ''Change,'' Sigel's latest finds him at his most vulnerable and his best.