perhaps its just that they don't want to be heard chatting instead of working by the boss?
do they talk normally at other times?
like peapod says get involved, start a chat, take some biccies or chocs to share with them or something.
if you stand there feeling paranoid and too scared to speak because you think they are talking about you, just think how that may appear to them...?
if they are not talking about you and you make no effort with them, they may just be thinking you are being rude, snotty, snobby, shy, perhaps even superior, whatever - but they won't like it.
perhaps one day suggest a quick drink after work?
bear in mind, sometimes you just click with people and feel very comfortable chatting away and feel on the same wavelength, with other people you don't - maybe they just feel like they have met a kindred spirit/friend for life, in this other woman and haven't even realised they are leaving you out.
you sound young and shy, so just be bold.