There are a number of posts hear from people that are or were cold during the MC. I for one get cold during my fasts/cleanses. I'm not sure if it's because the metobolism slows, there isn't heat generated from digestion, increased water in the system, etc.
i've no idea, exactly why, just happens.
One night I nearly begged my girlfriend to come over to keep me warm. On the phone she laughed and said "is that a line?" but when she came over she was like "WOW, I've never felt you this cold."