It's NOT gas! Pleeeese dont say it is - everybody knows what gas feels like, and we don't forget just 'cos we're pregnant! 
And i know perfectly well it's not the baby moving, as he/she is barely the size of a grain of rice. So ..... what could it be?
Here's what it feels like - rapid vibrations like a nerve twitching (but not quite as localised as it feels in outer parts of the body) all around my lower uterus area - like you've sat down on something that is 'humming' and gently vibrating through your seat!
It last for about 30 seconds or so - just long enough for me to stop what i'm doing and think 'there's that weird twitchy/vibration thing again..' and then it stops.
Sorry to sound touchy about the 'gas' idea, but when i looked at this question asked by other ladies on yahoo so many smug people have 'It's just gas darling, theres no way its your baby yet..' Dooooh!